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Unmanned Spaceflight.com _ Pluto / KBO _ Experts meet to decide Pluto fate
Posted by: MizarKey Aug 14 2006, 06:06 AM
One of many articles regarding the upcoming conference...
http://news.bbc.co.uk/2/hi/science/nature/4789531.stm
Posted by: paxdan Aug 14 2006, 07:11 AM
IMHO pluto is NOT a planet....
Just thought i'd kick off the inevitable debate
Posted by: akuo Aug 14 2006, 08:20 AM
I'm going to start a campaign to remove the planetary status of Mercury, if they drop Pluto. It's a glorified Vulcanoid!
Posted by: David Aug 14 2006, 11:31 AM
If we kick out Mercury, then we ought to do the same to Mars: it's closer in size/mass to Mercury than it is to Venus or the Earth, and its proximity to the main asteroid belt suggests that it should be considered merely a largish inner asteroid...
Posted by: Ames Aug 14 2006, 11:36 AM
Ok, Jupiter is a failed sub brown dwarf...
Nick
Posted by: djellison Aug 14 2006, 11:45 AM
You can think of all sorts of ways of branding when something is or isnt a 'planet'...but two things come to mind.
Does it actually matter? If we say Pluto is a planet or not, Pluto is still Pluto. Why waste the time, money and effort discusing this matter at all?
Whatever constraints you attempt to bring on the classification of a 'Planet' at some point you will have to make an arbritrary cut off point of size, shape and location and under various headings of planet, planetoid etc etc define ranges for each of these.
Doug
Posted by: ups Aug 14 2006, 12:12 PM
"About 3,000 astronomers and scientists are meeting in Prague to determine the fate of Pluto and the relevance of millions of schoolbooks and encyclopaedias around the world."
_________
Much ado about nothing ~ I think they're just going to Prague for a big party.
IMHO Pluto should remain a planet for historical sake if nothing else.
Posted by: rogelio Aug 14 2006, 01:06 PM
Concerning Pluto and the planet definition debate:
We have the same issues in biology. For example genus, species and every other taxonomic rank are, in the last analysis, arbitrary. And, yes, there are young hotshot biologists who want to scrap these categories entirely as being unscientific - and just go with cladograms (phylogenetic trees) when referring to and defining plants. But the result of such a proposal would be chaos in terms of how professionals would need to refer to plants (would a forester bother to describe an elm as the second distal branch on the third proximal Magnoliid clade, for example?). And I don’t even want to imagine how amateurs and schoolchildren would cope under such as system.
Same thing with “planets”. The concept is an arbitrary one, but ancient and culturally important and useful in maintaining interest and support for astronomy and space exploration. This is not a negligible consideration.
My solution (and the one I’m hoping comes out of the IAU meeting this week): Grandfather Pluto in as a planet. And any future discoveries (such as Xena) that are at least as large as Pluto (in diameter or mass, take your pick) become planets, too. Yes, we may ultimately have 2 or 20 more planets in the solar system, but won’t that be fun and create public interest and the impetus for more exploration?
Posted by: JRehling Aug 14 2006, 02:04 PM
[...]
Posted by: remcook Aug 14 2006, 02:46 PM
JRehling, that's one of the most sensible arguments I've heard in this eternal discussion :-)
Posted by: David Aug 14 2006, 05:22 PM
I agree that the divisions are arbitrary, that the term planet is "cultural" (or historical) rather than scientific, and I would also suggest that -- as we learn more about extrasolar systems -- classificatory systems that make sense in terms of our own solar system may be useless when discussing other systems.
However, in practical terms, the IAU as a nomenclatorial body has dug itself into its own ditch by having one set of naming conventions for "major planets" and another set for "minor planets" and TNOs: without ever stating what the distinction was. The fatal result of this imprecision is that the IAU was forced to make a determination on where the boundary between major and minor planets was as soon as an object larger than Pluto was discovered.
It would be helpful if the IAU would stress the limits of the decision that they are going to make: not that they are going to define what a planet is for all time, but that they are clarifying, for their own purposes, what the word "planet" means in terms of their rules of nomenclature, and that they cannot rule on how the word "planet" might be used in other circumstances.
Posted by: volcanopele Aug 14 2006, 06:47 PM
David brought up a good point that this is more procedural for the IAU than really anything else. Personally, I still feel that this argument over what is and what is not a planet is perhaps one of the most moronic arguments I have ever heard of. 
As others have mentioned, "planet" has no real scientific use.
I say let it be any natural object primarily (in other words not another planet, like Titan) orbiting a star. Yes, we have billions of planets. Do you have to memorize them all? Of course not. Since the voyagers flew past the giant planets, we have found moons that are just as interesting scientifically, if not more so. Can anyone here argue that Mercury is more scientifically important than Titan, Enceladus, Io, or Europa? But yet, because Mercury is given the gilded status of "planet" far more people are aware of Mercury and maybe a few of its properties, than they are aware of Titan or Io or any of the other interesting moons in our solar system.
And why are people having such a problem with Pluto being a planet or "Xena"? People can't contemplate a solar system with *shock* 10 planets? Did people in 1783 try to come up with definitions for planet to exclude Uranus? I mean 6 is a perfect number, there can't be more planets than the 6 known ones, obviously. So anything found outside the orbit of Saturn is a Trans-Saturnian object and not a planet, regardless of size. No, they accepted uranus as a planet, and its discovery spured on the hunt for another. Then Neptune's discovery spured the hunt for yet another.
If people want a size limiting definition, fine, go with the one rogelio suggested. Pluto is the lower limit for a planet. Anything found that is larger than it, is a planet. If there are 20 more planets, so be it. If there are thousands of potential planets, so be it. I'm not going to lose any sleep over it.
Posted by: rogelio Aug 14 2006, 07:08 PM
Yes, as volcanopele and others have mentioned, "'planet' has no real scientific use"
Yet... pure "science" is not completely the point here... It's possible that NH would not now be on its way to Pluto if there hadn't been that famous "Pluto - Not Explored" stamp in the planet postage stamp series.
Money, money for space exploration could be at stake. To me its seems appropriate and worthwhile for the IAU to decide the planet question. It's ivorytoweritis to deny the significance of the power of names and categories in influencing events that occur in the real world (like funding for space missions or astronomy research). The Pluto question has gotten a lot of play all over the world, and it does seem as if laypeople want a reasonable answer.
Posted by: DonPMitchell Aug 14 2006, 09:04 PM
An interesting point. Would NASA have been able to fund a probe to Pluto, if it had been downgraded in status?
I think if they are wise, the ITU will create a sensible definition for "planet" and then agree on a grandfather clause to keep Pluto. The debate is moronic, and it will probably turn ugly if they downgrade it.
Posted by: SigurRosFan Aug 15 2006, 11:18 AM
In the news ...
Tenth, and brighter than Pluto after that
Tom Gehrels, University of Arizona, USA
The regular asteroid observers, including amateur astronomers, are doing well with their CCDs in faint follow-up astrometry. However, large wide-angle telescopes and special equipment are needed to explore the outer solar system, including the rare objects that might qualify as planets. The searching is done with expensive telescopes by experts who are not always asteroid observers. The greatest encouragement for exploration of the outer solar system is the excitement that a new Planet might be found. Observatory directors and funding agencies are well aware of that.
This proposal is therefore to stay with the 75 years of popularly considering Pluto the Ninth, as the IAU agreed to in Manchester, and to adopt Xena as the Tenth Planet because it is intrinsically brighter than Pluto. The proposal is further that the same accurate and convenient criterion be used for naming an Eleventh Planet and so forth, namely that they be intrinsically brighter than Pluto, measured in “absolute V-magnitude.” Pluto's absolute visual magnitude is –0.76, Xena's –1.2. The present proposal is written on behalf of people who are doing the observing and discovering, who see the need for prompt recognition and the fastest return in naming. This has been explained before, in Nature 436, 1088, 2005 and Sky & Tel. 111, No. 1, 14, 2006, and this Letter has been circulated in draft form, but there has been no response from the two naming committees of the IAU. Considering roundness due to gravitational stability is complex, time consuming, subject to change, and impossible due to faintness at great distance.
A compromise for proper study and distinction of the various objects and populations is to attach to Pluto and to any new Planets also the usual comet or asteroid designation. Xena already has 2003 UB313, which eventually will be a 6-digit catalog number. The dual assignment, as Planet and comet or asteroid, will also stimulate discussion in schools and colleges of the rich variety of solar-system objects.
Posted by: Alan Stern Aug 15 2006, 11:43 AM
[quote name='volcanopele' date='Aug 14 2006, 06:47 PM' post='64281']
David brought up a good point that this is more procedural for the IAU than really anything else. Personally, I still feel that this argument over what is and what is not a planet is perhaps one of the most moronic arguments I have ever heard of. As others have mentioned, "planet" has no real scientific use.
I must say that I disagree. As reductionists, it is our job to categorize. Finding a workable definition
for a planet has only become necessary, and painful, because we have made so many fundamental
discoveries in our solar system and others since 1992 (the year the first KBO and the
first pulsar planets were detected). It's not about culture. It's about good science.
-Alan
Posted by: Greg Hullender Aug 15 2006, 01:47 PM
It's also worth mentioning that Ceres used to be called a planet, but once it became clear how many other bodies were in the asteroid belt, it lost that status. (Be interesting to learn exactly how that happened; I suspect there wasn't any kind of formal vote.)
Besides, there's a nice symmetry in having eight planets and two asteroid belts. The four terrestrial planets are inside the original asteroid belt and the four jovian planets are between that asteroid belt and the Kuiper belt.
Just tell the public they're not losing a planet -- they're gaining a new asteroid belt. I think most people aren't even aware there's a Kuiper belt at all.
Posted by: ljk4-1 Aug 15 2006, 01:49 PM
According to SpaceToday.net via NPR (National Public Radio):
A working group is expected to recommend to the International Astronomical Union (IAU) that Pluto be retained as a planet, opening the door for other solar system objects to also be designated as planets.
NPR reported this week that the group will likely report at an IAU meeting later this month in Prague that Pluto retain its designation as a planet, and that a new class of planets, perhaps called "dwarf planets", be created.
That class of planets would include Pluto, possibly the largest asteroids, and a number of the new large objects discovered in the outer solar system.
Pluto's classification as a planet has been questioned for the last several years as large icy bodies in the Kuiper Belt and beyond have been found. At least one of those objects, nicknamed Xena, is now believed to be larger that Pluto.
http://www.spacetoday.net/Summary/3452
Posted by: MahFL Aug 15 2006, 02:03 PM
I would like Pluto to remain a planet.
Posted by: ngunn Aug 15 2006, 03:56 PM
Does anyone feel like setting up a poll on this? (Idon't know how to.) There seem to be three positions:
1 The IAU should declare Pluto a planet.
2 The IAU should declare Pluto is not a planet.
3 The term 'planet' is scientifically obsolete and the IAU has no competence to decide on matters of wider word usage.
I go with number 3.
Posted by: volcanopele Aug 15 2006, 04:36 PM
for a planet has only become necessary, and painful, because we have made so many fundamental
discoveries in our solar system and others since 1992 (the year the first KBO and the
first pulsar planets were detected). It's not about culture. It's about good science.
But we also shouldn't present the solar system as a neat and tidy place when it isn't. The discoveries since 1992 have allowed us to appreciate the complexity of not just our solar system, but other solar systems as well. From other solar systems, we have found large planets that don't following neat and tidy orbits, some have high eccentricities for example. We have found stars with two accretion disks at different inclinations. In our own solar system, we have found icy dwarf bodies that follow a miriad of orbits and have various shapes, and there maybe some the approach the size of the terrestrial planets.
The solar system (and other systems) are not neat and tidy places and we shouldn't pretend that it is. Listen, I understand we need a system for categorization. It allows us to more easily make sense of our world or the worlds around us. I understand that. But the amount of press this has gotten and the amount of breath and time spent on this is not worth it. Pluto is still Pluto whether it is a planet or a TNO, or any icy dwarf, or a dog.
Setting arbitrary definitions also makes the word less useful for scientific purposes. A TNO at 4000 km probably didn't form fundimentally any different from a 2000 km wide body (or a 1900 km wide body). As long as we make it clear to the public what the words value is (for classification purposes and for nomenclature purposes), I think we can come to an understanding. But if we treat it as if objects that are planets are some exclusive group or club and those that are just moons or minor planets are inferior and aren't worth our time in terms of exploration purposes (just because they are not planets), then we have a problem.
Okay, I'm sorry about the rant...
Posted by: AlexBlackwell Aug 15 2006, 04:45 PM
Jason, I think you and everyone else are missing Alan's point. No one is trying to obscure the fact that our "solar system [isn't] a neat and tidy place." Quite the contrary. Taxonomies and classification systems are very useful in science, especially in astronomy. Discerning hierarchical relationships, ipso facto, can lead to scientific discoveries.
Posted by: David Aug 15 2006, 04:56 PM
You're right, there wasn't. The following historical summary discusses how it happened:
http://aa.usno.navy.mil/hilton/AsteroidHistory/minorplanets.html
The decision was in the hands of the compilers of astronomical almanacs. For the first 50 years after the discovery of Ceres, asteroids were listed together with the planets, between Mars and Jupiter, in order by length of their semi-major axes. In 1841 the British Nautical Almanac and Astronomical Ephemeris began to collectively name the four then-discovered asteroids as "Minor Planets". In 1851 asteroids 5-15 and Neptune were moved "to the back of the book" of the Berliner Astronomisches Jahrbuch. At the same time, numbers were substituted for the astronomical symbols that had been invented for them. In 1867, Ceres, Pallas, Juno and Vesta went "to the back of the book" as well. Likewise, in 1868, the Paris Observatory began to classify these four asteroids as "petites planètes". So for a few decades, at least, there were three categories of planet: major (Mercury-Neptune), minor (asteroids 5+) and a nameless middle group consisting of asteroids 1-4. In the '50s and '60s other almanacs also stopped printing the ephemerides of the asteroids in the same section with the planets or abandoned them altogether.
The distinct treatment of Ceres, Pallas, Juno and Vesta depended not so much on their size (though Ceres, Pallas, and Juno were drastically overestimated) but, I think, on the fact that they had been treated as planets for forty or fifty years -- in a situation comparable to that of Pluto today. They were, you might say, "grandfathered in". Astraea and the others were (c. 1850) newcomers, only discovered in the past decade, and so not worthy of the same degree of reverence!
If Pluto, 2003UB313 and some others are granted a middling status like "mesoplanet", perhaps we can expect them also to drift into being merely "minor planets" some decades from now.
Posted by: volcanopele Aug 15 2006, 04:56 PM
I understand that. I guess my point was that given the current proposals (with the exception of the roundness one) are arbitrary and don't use anything fundamental about the body itself to seperate "minor" from "major". Based how much this is argued and how much press this gets, you'd think that this was something more important, but really, it isn't. Discerning hierarchical relationships is very important, I grant you, but most of the proposals don't do that.
Personally, I prefer a definition that is inclusive rather than exclusive. As the article by Tom Gehrels stated, such an inclusive defintion (which would be one that would allow for the discovery of "planets" in the Kuiper Belt, rather than excluding all members of that region of the solar system), would help to spur future scientific discoveries and would help boost research into the outer reaches of the solar system.
Posted by: David Aug 15 2006, 05:01 PM
I don't disagree; but if it were the business of the IAU to try to make its nomenclatorial system conform to any one of several possible planetary taxonomies, surely the first order of that business would be to find a way of pointing out that Jupiter and Mercury are not the same kind of object?
Posted by: Alan Stern Aug 15 2006, 05:36 PM
Perhaps these will help some who have not seen them; sorry for spamming those who did-- the
links will save me from typing my views:
Gravity Rules: http://www.spacedaily.com/news/outerplanets-04b.html
Copernicus Smiled: http://www.thespacereview.com/article/450/1
Posted by: David Aug 15 2006, 05:58 PM
links will save me from typing my views:
Gravity Rules:
I'm quite fond of the "rounded by gravity" criterion myself; but the presence of objects like 2003 EL61 and Iapetus makes it rather difficult to apply. Objects with diameters between 400km and 1600km exhibit a wide variety of shapes: spheres, near-spheres, flattened spheroids, spindly spheroids, nicely rounded ellipsoids, bumpy, lumpy, and partially concave ellipsoids, and plain old irregulars. If there's a direct correlation between shape and size or mass, it's not an obvious one.
Why wouldn't a cutoff above 1600km diameter be just as defensible a gravity-based division as one below 400km?
Posted by: Alan Stern Aug 15 2006, 06:23 PM
Why wouldn't a cutoff above 1600km diameter be just as defensible a gravity-based division as one below 400km?
Careful, careful, careful! The roundness argument is not about whether an object is round or not-- because it could for axample be tidally bulgded or rotationally distorted. I's about whether its massive enough **Tto be rounded by gravity** in the absence of
the other effects. You will see this speccifically noted in the IAU language tomorrow.
-Alan
ps. EL61 is probably not a big egg: it's most likely a huge contact binary. At least that's where my money is.
Posted by: David Aug 15 2006, 06:37 PM
I realize that there is this "out", but it seems to me that it makes the concept very nebulous and subject to a lot of special pleading. One might argue for any number of objects below 400km diameter, right down to the size of "roundable" water droplets, that they could be gravitationally rounded in some ideal situation, but due to a variety of other factors (like impacts) they don't happen to have fully realized their inner roundness.
That's the argument used for Vesta, for instance: it would be a nice clean spheroid if it weren't for that rotten polar impact crater!Interesting thought.
And for the sake of amusement, the author of a http://astronomy2006.blogspot.com/ from Prague http://astronomy2006.blogspot.com/2006/08/seed-gets-it-wrong.html that:
Posted by: Alan Stern Aug 15 2006, 07:31 PM
That's the argument used for Vesta, for instance: it would be a nice clean spheroid if it weren't for that rotten polar impact crater!Interesting thought.
And for the sake of amusement, the author of a http://astronomy2006.blogspot.com/ from Prague http://astronomy2006.blogspot.com/2006/08/seed-gets-it-wrong.html that:
Let's discuss this tomorrow after you see what the IAU position is. Some of your questions will be addressed.
-Alan
Posted by: ljk4-1 Aug 15 2006, 08:13 PM
"Seed magazine links here, but predicts that you will be able to find out if Pluto is a planet here. No, you won't! I think this is an incredibly unimportant topic, it's not what this meeting is about and I will not mention it at all."
Ah, an intergalactic snob from the old days of astronomy. You know, the further it
is from the Sol system (meaning Percival Lowell and his ancient Martians with their
darn canals), the more important it is - to the professional astronomers.
Instead of this elitist attitude, I hope astronomers will use this opportunity to
educate the public and media on our favorite science while at least one aspect
of the field is hot, trendy, and generating publicity.
As for naming Pluto and all the smaller worlds, what about the good old term
Planetoid?
It means "little planets", is less awkward than dwarf planets, and goes in
line with the less accurate term asteroid (little star).
I also think black holes should be called collapsars, keeping in line with
pulsars and quasars.
Posted by: JRehling Aug 15 2006, 08:52 PM
[...]
Posted by: AlexBlackwell Aug 15 2006, 09:03 PM
What many people who have a sentimental attachment to the term "planet" lose sight of is the fact that the term originated in ancient times to describe the appearance of certain "wandering stars." There was absolutely no scientific need for it. Therefore, despite the very long usage of "planet" and what it has come to stand for, I don't have any particular qualms about redefining it, for example, to take into account our rapidly growing base of knowledge about Edgeworth-Kuiper Belt objects. And hey, if http://solarsystem.nasa.gov/people/profile.cfm?Code=SternA ever find the hypothesized population of Vulcanoids, then I have no sentimental attachment in reclassifying Mercury, as well.
Posted by: DFinfrock Aug 15 2006, 10:58 PM
So does that mean that as a major comet brightens on approach to the sun, it magically transmogrifies into a planet? There has to be more to the definition than that.
David
Posted by: volcanopele Aug 16 2006, 12:09 AM
David
Obviously they mean the absolute visual magnitude of the body itself, not any associated coma or other debris cloud.
Posted by: dvandorn Aug 16 2006, 03:40 AM
I think that the concept of "planet" has really nothing to do with the scientific hierarchical classification system. As far as that goes, I think we have five major classifications -- ISSRO's (Inner Solar System Rocky Objects), GGO's (Gas Giant Objects), IGO's (Ice Giant Objects), KBO's (Kuiper Belt Objects) and OCO's (Oort Cloud Objects). A separate set of classifications can be applied to moons of these objects.
I will restate, though, that when an average citizen of the Earth asks "What are the planets?" he/she is asking something similar to "What's the layout and population of my town?" They don't want to gain new scientific insights into the Solar System, they want a number and a set of names they can wrap their minds around, feel comfortable with, and go out armed with the knowledge that they at least know the basic layout of their own little corner of the Universe.
For example, there was a time when the population of a town was only expressed in the number of adult white males that lived there. Then women and minorities made it clear that they needed to be counted, and so the concept of what made up the census of people in a given place changed. What we're arguing about here is similar to the little old lady who complains that the census says she lives alone, when she actually lives with her fourteen cats, and she demands that the cats be counted in...
In other words, your average person, in my humble opinion, doesn't care about the fine scientific distinctions. They want to know the names of the streets in their neighborhood, the names of the families that live nearby, and where City Hall, the grocery store and the shopping mall are located. They don't want (or need) to have their "naming of things" stretched out to include detailed numeric representations of every street, path, walkway, sidewalk, and alley, nor do they have any need to know the names of every cat, dog, gerbil and flea that lives near them. If you give them such a detailed accounting, they will simply ignore it. They will know it exists, but they just won't care.
I think that's why this whole issue with Pluto is getting some people energized. They don't really care why something is named a planet or some other thing, they want to know the equivalent of the street names in their town and where their friends and acquaintances live. They want to know the names of, and a little about, the "places" in our Solar System, and if that list grows from 9 to 256,347, they're going to ask for (and get!) a list of just those places they ought to consider "important."
In the end, it's that list they ask for -- the one that defines the "important" places in the solar system -- that will be the list of the "planets." At least, it will be the only list that anyone beyond a small handful of scientists will ever memorize or feel that they "know"...
-the other Doug
Posted by: Holder of the Two Leashes Aug 16 2006, 03:45 AM
It has been announced on the SpaceDaily website that the committee was unanimous in defining a "Planet" as a body big enough to round itself off gravitationally, and whose shape is determined by hydrostatic rather than rigid forces.
Pluto remains a planet under this definition.
All seven members of the committee are reported to be in complete agreement on this. And this will be the draft submitted to the IAU.
They report that there are twelve known planets in our solar system under this definition.
Posted by: mcaplinger Aug 16 2006, 04:05 AM
Spacedaily claims that Charon makes the cut as a planet. I don't see how, since the body has to be in orbit around a star. It seems like they are saying that if the barycenter is outside either body (or something like that) then both bodies are planets.
Seems kinda silly to me. Worst. Definition. Ever.
Posted by: volcanopele Aug 16 2006, 04:05 AM
? only 12? In your other post, you stated that they considered this size to be around 850 km. So in addition to the current 9, I assume you would have 2003 UB313, Ceres, and Sedna, at least. But I seem to recall several other currently known bodies in the Kuiper Belt larger than 850 km across, such as Quaoar, 2005 FY9, and 2003 EL61 as well as perhaps 2002 TC302.
I am very happy to see that the definition has some basis in the physical nature of the body and not some arbitrary cutoff, like 2000 km. While it may take some time for people to accept that Ceres will now be called a planet, I am glad that this whole non-sense is finally nearing an end and we can all get on with our lives.
EDIT: Okay, I think we can all agree that Charon is not a planet...
Posted by: dvandorn Aug 16 2006, 04:23 AM
So, is the new nursery-rhyme mnemonic for the planets going to go something lik this?
"My Very Educated Mother, Catherine, Just Served Us Nine Pickled, Spicy Xylophones."
-the other Doug
Posted by: nprev Aug 16 2006, 05:11 AM
Mmm...pickled spicy xylophones....
Interesting take from the IAU on this contentious issue. Ceres, though...looks pretty spherical from the HST images (which Dawn should amply confirm), as does Vesta despite its south polar divot...gotta wonder what Chiron's morpology is , as well as that of other Centaurs, and where to really draw a line that might mean something?
Point here is, obviously, that if this definition is rigorously enforced we may end up with a heckuva lot of planets, and that might get pretty cumbersome very quickly. Why not add a diameter provision (let's say that of Pluto, because it's probably a safe bet that there are many bodies bigger than 2003UB313 further out) as an additional requirement for planetary status? Like everything else in nature, the natural satellites of any star will exhibit a continuum of sizes rather than fall into nice, neat categories...gotta put this fire out now before it spreads.
And actually, let me present a new concept: the "Mercury Standard". Mercury is the smallest universally recognized planet, and its distinctive features with respect to planethood are that it formed independently as the result of accretion from the primordial Solar nebula at or near its present orbit. Using this logic, any body that is smaller than Mercury and in an orbit that exhibits any evidence of prior association with a major planet (the old "escaped moon" chestnut) would be excluded from planetary status. This would presumably eliminate Pluto, but perhaps admit Sedna and 2003UB313 if their respective orbital parameters are "clean".
Posted by: volcanopele Aug 16 2006, 05:22 AM
I certainly have no problem with having a lot of planets. I say, the more, the merrier (Though if I had my way, we would have a couple of hundred thousand known planets, but I digress). I am just happy that some arbitrary cutoff wasn't chosen.
Personally, I am happy to see the inclusion of Ceres, though I hope that the objects I listed are not excluded and the list of bodies given by Spacedaily are just there extrapolation from the definition (particularly with the Charon inclusion, God I hope they meant to say Sedna).
Though the more I read, the more confused I am. What is this "pluton" non-sense? Will this have the same effect as "gas planet" and "terrestrial planet", or is this just an attempt to keep our heat-impaired comrades down? The idea of 8 "classical" planets bothers me a bit, again since it is a "exclusive" definition.
Posted by: DonPMitchell Aug 16 2006, 06:25 AM
A wise choice. They have a nice physically-based definition instead of "I think Mercury is big enough, but Pluto isn't". Well worth the millions spent on their big meeting in Prague.
If they had reduced us to 8 planets and kicked out Pluto, many people would surely have been upset. It would have been hard to get concensus at IAU, which undermines it. And it would be harder to get funding to look for and study new Planets in the outer solarsystem.
[attachment=7015:attachment]
But instead they gave us three new planets. TV news programs tomorrow get to tell everyone there are 12 planets, they get to introduce them to Ceres and explain it is named after the goddess of wheat or whatever. And nobody is going to be angry, because who has a grudge against Ceres?
[attachment=7016:attachment]
So now the next pointless but irresistable debate will certainly be, what do we call 2003UB313?
Posted by: djellison Aug 16 2006, 08:28 AM
Totally unrelated politics and political imagery removed. You all know the rules guys.
Doug
Posted by: MichaelT Aug 16 2006, 09:06 AM
The relevant IAU press release can be found here:
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html
Yes indeed. This is an excerpt from the news text:
"For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet."
Michael
Posted by: ngunn Aug 16 2006, 10:31 AM
I wonder what happens if the mutual orbits are eccentric and the barycentre moves in and out of the larger body every 'month'?
Posted by: djellison Aug 16 2006, 10:39 AM
And just for good measure, an article about a backward polarity sun spot today describes sun spots as being 'planet sized'
PLANET SIZED
What the hell does that mean
Doug
Posted by: ugordan Aug 16 2006, 10:50 AM
Perhaps an "average" barycenter point can be taken as measure in that case. Say circularizing the orbits, leaving the orbital periods constant?
Posted by: paxdan Aug 16 2006, 11:06 AM
What the hell does that mean
Pah! that's an easy one: http://en.wikipedia.org/wiki/1_Ceres to http://en.wikipedia.org/wiki/Jupiter km.
Posted by: JamesFox Aug 16 2006, 11:07 AM
Personally, I have to admit that I feel rather uneasy about this proposal. People are liable to reject it on unscientific grounds because it provides 'too many planets'. Also, why do they mention only three planet candidates to the news, while treating other qualifying objects in a a separate, not-mentioned to the news category? I've seen quite alot of opposition already.
I think a slightly more acceptable definition would stress the difference between the 'dwarf planets' and the 'eight classical planets', thus allowing those who are so inclined to ignore the dwarf planets, while the inclusivists would include the dwarf planets.
Posted by: David Aug 16 2006, 11:15 AM
My recommendation to anyone who has to talk to the press about this is:
1. Design a wallet-sized card that lists all the objects in the Solar System that will be labelled "planets" under the new definition: don't go overboard, but include name and a couple of basic facts, like diameter and "distance from the Sun".
2. Print out several hundred copies of this card.
3. Hand it out/ e-mail it to everybody who asks (and everybody who doesn't).
Posted by: JamesFox Aug 16 2006, 11:25 AM
The problem with this is that the sizes of many objects are currently very uncertain. How big is Orcus, Sedna, or Varuna? Estimates have been made, and some objects are certainly big enough despite uncertainties in size, but I'm not sure how the public will take to such probably long-lasting indeterminate statuses. It will take years before even the objects currently known have thier sizes determined accurately enough.
Posted by: David Aug 16 2006, 11:34 AM
That's why God invented the asterisk.
Posted by: Ames Aug 16 2006, 11:40 AM
That sounds sensible to me and something that the public could understand.
But they are going to have a field day with barycenter...
"What? Barry Sentor? - never heard of him!"
I think Pluto should be a planet(maybe dwarf maybe -oid) with a large(in comparison) moon that just happens to be large enough to set the "Barry Sentor" above Plutos' surface. Why a double planet?
We either need to make it a simple definition that the Public will understand and accept, or a rigorous (and useful) definition for scientists. Otherwise I fear that the Public may just ignore the more difficult concepts and revert to the old definition of the solar-system and the scientsts will be off in their own word of planetoids, plutinos, barrycenters...
Nick
Posted by: Ames Aug 16 2006, 11:44 AM
But they are going to have a field day with barycenter...
"What? Barry Sentor? - never heard of him!"
I think Pluto should be a planet(maybe dwarf maybe -oid) with a large(in comparison) moon that just happens to be large enough to set the "Barry Sentor" above Plutos' surface. Why a double planet?
We either need to make it a simple definition that the Public will understand and accept, or a rigorous (and useful) definition for scientists. Otherwise I fear that the Public may just ignore the more difficult concepts and revert to the old definition of the solar-system and the scientsts will be off in their own word of planetoids, plutinos, barrycenters...
Nick
Actually Barry Senter
www.barrysenterdesign.com
Hmmm!
Nick
Posted by: David Aug 16 2006, 11:57 AM
Does "double planet" imply a single entity that happens to consist of two units, or two planets that happen to be revolving around each other?
In other words, can one say "Pluto-Charon is a planet"?
Posted by: MichaelT Aug 16 2006, 12:07 PM
In other words, can one say "Pluto-Charon is a planet"?
I don't think so. The IAU release explicitely states 12 planets including Charon. If the double planet was counted as one entity, there'd be only 11. So they are two planets revolving around each other.
Michael
Posted by: ugordan Aug 16 2006, 12:25 PM
Hmm... We seem to be running in circles here, so to speak. Didn't they say a body needs to orbit the Sun, not another body, in order to be classified as a planet? If so, how can Charon (and for that matter Pluto as well!) be a planet?
Posted by: paxdan Aug 16 2006, 12:41 PM
Barry Sentor would like a word
Posted by: ngunn Aug 16 2006, 12:44 PM
"My Very Educated Mother, Catherine, Just Served Us Nine Pickled, Spicy Xylophones."
-the other Doug
How about: Many Vexed Experts Make Confusing Judgment So Us Normal People Say 'XXXX'
Posted by: ljk4-1 Aug 16 2006, 12:48 PM
Just how binding is the IAU decision on astronomers, both professional and
amateur?
Does it have to be taken any more seriously than, say, the UN Outer Space
Treaties are?
People are already buying up lunar property. Just wait until corporations start
landing there to mine the regolith and see how quickly and easily their lawyers
circumvent that dated bunch of rules.
As for an example right here on Earth, the Antarctic Treaty is frequently
violated and ignored by the numerous countries which claim various sections of
the southernmost continent, which they have sliced up like a pie. There are even
ongoing disputes over who owns certain parts of Antarctica.
In their efforts to keep Pluto an "official" planet, the IAU has made the issue even
more complicated for future generations. We still know so little about "Xena" and
you know there are even bigger worlds out there just waiting to be found and
argued over.
And what about all those objects orbiting other stars?
They should have gone with planetoids.
Posted by: rogelio Aug 16 2006, 12:51 PM
Dateline 2015: U.S. Postal Service issues revised planetary exploration postage stamp series ending with “Xena –Not Yet Explored (or formally named)”...
Posted by: Ames Aug 16 2006, 12:53 PM
I like it
Nick
Posted by: Alan Stern Aug 16 2006, 12:57 PM
Owing to its high mass relative to Pluto, Pluto-Charon's barycenter is in free space,
which means Charon is technically in orbit about the Sun, not Pluto. This is not a debatable
fact, it's just the way the dynamics works.
Posted by: ngunn Aug 16 2006, 01:22 PM
which means Charon is technically in orbit about the Sun, not Pluto. This is not a debatable
fact, it's just the way the dynamics works.
The barycenter of the solar system is also in free space. Does this mean that technically Jupiter is in orbit about the Milky Way, not the Sun?
Posted by: maycm Aug 16 2006, 01:25 PM
My kids have a video of "Blues Clues" where 'Steve' sings the following song to teach them about the planets.
Oh the Sun's a hot star
And Mercury's hot too
Venus is the brightest planet
And Earth is home to me and you
Mars is the red one
And Jupiter's most wide
Saturn's got those icy rings,
and Uranus spins on it's side
Neptune's really windy And Pluto's really small
Well we wanted to name the planets and now we named them all.
...seems it will need updating. Any suggestions? I know there are some creative people here 
Posted by: ugordan Aug 16 2006, 01:25 PM
which means Charon is technically in orbit about the Sun, not Pluto.
Why would that mean Charon is technically orbiting the sun? What difference does it make whether or not the barycenter is below the surface of the primary body? What's so special about surface radius, dynamically speaking?
Shouldn't the fact which object exerts a greater force on the moon determine what it's technically orbiting? Similar to the case with our Luna -- I don't know the numbers or if it's exactly true, but it's been stated the Sun exerts a greater pull on it than Earth does. So technically the Moon orbits the Sun.
This barycenter-based definition sounds pretty vague and useless to me.
Posted by: David Aug 16 2006, 01:29 PM
which means Charon is technically in orbit about the Sun, not Pluto. This is not a debatable
fact, it's just the way the dynamics works.
Of course Charon is in orbit around the Sun, but then so is Earth's Moon.
Here's another way to look at it: if you sketch the ellipse of Charon's orbit about the Pluto-Charon barycentre, Pluto is always within Charon's orbit; if you sketch the ellipse of Pluto's orbit, Charon is never found inside it:
Pluto's orbit is so close to the barycentre that saying that Charon doesn't orbit Pluto (but rather a bit of empty space close to Pluto), though technically correct, seems whimsical, sort of like denying that Jupiter revolves around the Sun.
Posted by: djellison Aug 16 2006, 01:37 PM
I must admit - I was explaining all this to my ever fascinated colleague Josh ( for those that watch QI I said "How many planets are there Josh" "Nice?" "BZZZZZZZZZ -awooga...awoooga" ) and we debated the point of Charon getting an upgrade.
Why should the movement of the barycenter from 1m below the mean radius to 1m above reclassify the system from being planet + moon to being binary?
Systems evolve, that reclassification could occur during the lifetime of a system, and I don't think a body should be reclassified from moon to planet just because it got a little further away.
A body is what a body is....it doesnt matter where it is (as the rest of these new rules would have us believe)...and I think it makes a bit of a farce of the system if suddently we have to go back to our text books because a system has evolved and scrub out 'moons...1' and instead right "binary planet"
BUT....then...there ARE likely to be binary systems out there, so at what point do you say "ok - this is now a binary system, not planet+moon" - mass, radius, etc etc....you do need a cut off, but I don't think it should be one that can change.
Doug
Posted by: Tom Tamlyn Aug 16 2006, 01:44 PM
Today's New York Times has a good http://www.nytimes.com/2006/08/16/us/16pluto.html?hp&ex=1155787200&en=e7610d06372fa9bf&ei=5094&partner=homepage on the controversy, with quotes from Alan Stern, as well as an http://www.nytimes.com/2006/08/16/opinion/16brown.html?_r=1&oref=slogin by Mike Brown.
TTT
Posted by: David Aug 16 2006, 01:49 PM
I'm just imagining:
1) A "lumpy" primary (shaped something like Iapetus) where the barycentre is sometimes below the actual surface and sometimes above it
2) An ellipsoidal primary, where the barycentre is sometimes below the actual or notional surface of the ellipsoid and sometimes above it
3) A primary with a thick atmosphere, where the barycentre is suspended somewhere "in the middle of the air"
4) A primary with an atmosphere whose height changes seasonally, so that the barycentre is sometimes in the atmosphere and sometimes in space
You can have all sorts of fun with this, the more so if you happen to live on such a world. "Is that the Moon?" "No, it's a planet... today..."
Posted by: ngunn Aug 16 2006, 02:00 PM
Nick
Grand! However I now realise my mnemonic needs editing. I was following 'the other Doug' who inadvertently inserted an S (for Sharon?) instead of a C for Charon. My definitive version is now as follows:
Many Vexed Experts Make Confusing Judgment So Us Normal People Cry 'XXXX'
The wor(l)d denoted by XXXX has not yet been fully explored.
Incidentally I see that the word 'farce' has just entered this discussion. . . quite so.
Posted by: Greg Hullender Aug 16 2006, 02:02 PM
It does seem that it would have been useful to create a more general term (e.g. planetoid) to cover any non-fusing body rounded by its own gravity (absent effects of rotation or tides) but ignoring its orbital dynamics. Eath, Luna, Ceres, Titan, etc. would all be planetoids. Everything smaller could be an asteroid (or maybe a new name), while everything larger would be a star. Only a truly transforming cataclysm could transform one into another.
A multiple planetoid would just be any long-term stable, gravitationally bound set of planetoids. (Likewise you could have a multiple asteroid I guess.)
The planet vs. moon distinction still comes down to whether one member of the system sufficiently dominates the rest. I suppose the barycenter definition is as good as any for this purpose, although it bothers me that it depends on the density of the planet(oid).
This also suggests that we might usefully make a distinction between a "moon," which would have to be a planetoid, and a "moonlet" (is there a better name) which is just an asteroid that's gravitationally bound to a planetoid.
Posted by: mcaplinger Aug 16 2006, 02:33 PM
The barycenter rule is laughable, IMHO. You'd think they could have tried a little harder if the intent was to handle extrasolar double planets in the future, unless somebody had some political agenda to make Charon a planet. I'd have tried to make the minimum barycenter distance some function of the body radii so as to exclude Charon.
I also wonder how well the hydrostatic rule will work in practice around the low end, something we are likely to see either for KBOs or even for the larger asteroids.
Leave it to the IAU to overcomplicate what was a seemingly simple question.
Posted by: ljk4-1 Aug 16 2006, 02:42 PM
A camel: A horse designed by committee.
Posted by: alan Aug 16 2006, 03:04 PM
They can't change Pluto's clasification without getting hate mail from millions of children so they had to come up with another definition. Once Quaour's size has been confirmed by stellar occultion it will be added too. Then the kids will have to learn how to pronounce Quaoar, that'll fix them.
Posted by: David Aug 16 2006, 04:25 PM
Let me suggest instead that the criterion should be in terms of ratio of the smaller distance to the barycentre / the total distance between the gravitational centers of the two objects -- regardless of the radius of either of the two bodies. That ratio would be the same as the ratio of the smaller of the two bodies to the combined mass of the system.
A "perfect" double planet would consist of two objects of the same mass with the barycentre halfway between them. Nobody would quarrel with that being called a double planet.
A possible criterion for a double planet system might be something like: a system in which the distance from the gravitational center of the smaller body to the barycentre is at least one third of the total distance between the gravitational centers of the two bodies -- in other words, the mass of the smaller body should be at least half the mass of the larger one.
Posted by: JRehling Aug 16 2006, 04:29 PM
[...]
Posted by: jsheff Aug 16 2006, 04:35 PM
Results are in:
http://www.space.com/scienceastronomy/060816_planet_definition.html
Posted by: maycm Aug 16 2006, 04:40 PM
http://www.space.com/scienceastronomy/060816_planet_definition.html
....well not yet.... they vote Thursday 24th August
Posted by: AlexBlackwell Aug 16 2006, 04:46 PM
I'm not sure if this has been mentioned, but http://www.washingtonmonthly.com/archives/individual/2006_08/009347.php's Kevin Drum's take on the issue.
Posted by: vexgizmo Aug 16 2006, 04:51 PM
"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape...."
The "roundness" criterion seems to me to be the start of a good definition. And if Pluto is a planet so is Ceres. However, the qualificaitions of "dwarf planet," "pluton," and planet-if-beyond-the barycenter seem extraneous. But there is a problem here. Keep in mind that we have no idea if Callisto is truly in hydrostatic equilibrium, so surely we have no idea about 2003 UB313. In reality the definition can't really be of "hydrostatic equilibrium" without defining an error bar. This seems pedantic but becomes a real issue: does Vesta count? Before its large impact, its shape was probably hydrostatic (cf. Thomas et al., Science 277, 1492 - 1495, 1997). Is Vesta penalized just for being whacked with an impact large compared to its radius?
Posted by: David Aug 16 2006, 05:14 PM
Ultimately all taxonomic systems have "marginal cases" problems. That's not a flaw in the concept of taxonomy, it just comes with the territory. What's a little odd, however, is to see one taxonomic criterion denounced for having marginal cases, or depending upon precise measurement, while another taxonomic criterion -- that also has marginal cases problems -- is proposed as a replacement.
Anyway, while the IAU's proposal creates a framework for discussion, I don't expect it to be the last word; I imagine that, whatever the IAU decides on, it will be revisited many, many times in the years to come, as more and more data better defines the shape of the problem.
This seems pedantic but becomes a real issue: does Vesta count? Before its large impact, its shape was probably hydrostatic (cf. Thomas et al., Science 277, 1492 - 1495, 1997). Is Vesta penalized just for being whacked with an impact large compared to its radius?
Alan Stern discussed this above (post #28 above, with my reply following). One of the reasons I remain focused on actual roundness as opposed to essential or intrinsic or original or probable roundness is that it has at least one practical application in terms of planetary cartography: in those terms a (really) round object is one that you can map using cartographic tools and techniques developed for mapping the earth without extensive error, and an irregular object is one that requires the use of exotic shape models and grids. I don't know exactly where that line would be drawn, but you could try asking Phil Stooke.
Posted by: hendric Aug 16 2006, 05:29 PM
The correct answer for the marginal cases is simply making an edict from on high, after a suitable amount of time researching the size. For example, KBO 2010 XYZ112 is bright enough it's albedo could let it be greater than the cut off. Wait a decade (seems like a reasonable time limit), then if there still is uncertainty, pronounce an edict.
Posted by: AlexBlackwell Aug 16 2006, 05:31 PM
That's very true, and very legalistic, too. In fact, my wife, who happens to be a lawyer, quoted Justice John Paul Stevens from the http://www.supremecourtus.gov/oral_arguments/argument_transcripts/03-633.pdf in http://supct.law.cornell.edu/supct/html/03-633.ZS.html:
QUOTE...but the purpose of a [constitutional] bright line test is to avoid litigation over the borderline cases...
Posted by: AlexBlackwell Aug 16 2006, 05:44 PM
The editorial that appears in the http://www.nature.com/nature/journal/v442/n7104/index.html:
QUOTEEditorial
Nature 442, 719 (17 August 2006) | doi:10.1038/442719a; Published online 16 August 2006
Round objects
Planets are spherical, and the International Astronomical Union's attempt to make this part of their definition has merit.
There was once a prissy British civil servant who, when he came across a passage in a memo that displeased him, wrote "round objects" in the margin as a synonym for something ruder. This arch circumlocution was lost on the bluff minister he served, who fired back a query as to who this Round fellow was, and why he objected so much.
We can expect there to be plenty of members of the International Astronomical Union (IAU) who, reading the proposed new definition of a planet offered to them by their executive committee, will want to scrawl something equally rude and rather blunter in the margin — and will want to make their objections heard, possibly quite vociferously, at their general assembly in Prague this week (see page 724).
We understand and, to some extent, sympathize. But we would suggest that, instead, they acquiesce in the new definition, which will have the effect of increasing the number of planets in the Solar System to 12, and open the doors to more. They should do this for two reasons: it is not a bad definition; and it will at least stop the rumbling debate over the status of Pluto.
In the 1990s, it became clear that Pluto, the most newly discovered planet, was the most conspicuous of a crowd of icy 'trans-neptunian objects' (TNOs), some of which might well be larger. There was an obvious historical parallel to this situation with asteroids in the nineteenth century. When it was found that there were dozens of asteroids, Ceres, the largest and first discovered, was demoted from its position as a proper planet; it is now a 'minor planet' along with all the other asteroids. Pluto, it was argued by analogy, should be a minor planet with the rest of the TNOs on similar grounds.
This proposal sparked a degree of public debate that irritated many astronomers, who felt that the question of whether a particular body gets called a planet or not is of no scientific interest whatsoever. Still, the IAU decided that it should try and resolve the matter: planets loom large in the public imagination, and it seemed only reasonable for astronomers to be able to say whether a new discovery (or for that matter an old friend) was a planet or not.
The IAU's proposal is that the term 'planet' should apply to an object that has a sufficiently strong gravitational field to have pulled itself into a spherical shape, that is in orbit around a star, but that is not a star itself. This lets in Pluto and 2003 UB313, a TNO that is a touch bigger and not yet equipped with an IAU-approved name. It also readmits Ceres. And in the most peculiar aspect of the whole business, Charon, previously considered to be a moon of Pluto, will become a planet in its own right. Moons, however spherical, will remain satellites, not planets, in the IAU's eyes. But because the centre of mass of the Pluto–Charon system lies outside the body of Pluto, Charon, although tiny compared with, say, Neptune's moon Triton, qualifies as a planet.
Nine more TNOs, and three more asteroids, will become candidate planets, pending further investigation of how spherical they are. More planetary TNOs may follow, when discovered. To tidy things up, the minor planets will get renamed: those that don't have enough of a gravitational grip on themselves to be proper planets will now be 'small Solar System bodies'.
All this will doubtless lead to ructions. But it is at least a coherent approach, and it has a fairly clear basis in physical properties. It has been convenient to have a small and easily memorized number of planets in the Solar System, but convenience is not the only thing that counts. The effects of mass define (unofficially) the upper limits of the planetary realm; anything big enough for fusion is a star. It is fitting, then, that mass should define the lower limit too. This, we think, adds up to a case for IAU members to accept the proposal.
Posted by: JRehling Aug 16 2006, 05:49 PM
[...]
Posted by: Alan Stern Aug 16 2006, 05:51 PM
[
> DPS PRESS RELEASE (Released 16 August 2006)
>
> "Planetary Scientists Support Proposed Redefinition of a Planet"
>
> Recent discoveries of objects in the outer reaches of our Solar System
> have forced scientists to reconsider what it means to be a planet. The
> International Astronomical Union (IAU) has proposed a new definition of a
> planet as a celestial body whose gravity is strong enough for it to be
> nearly round in shape and which is in orbit around a star but is itself
> neither a star nor a satellite of a planet. According to this definition,
> the nine traditional planets in our Solar System would be joined by Ceres
> (the largest of the asteroids), by Charon (Pluto's largest moon), and by
> 2003 UB313 (the provisional name for a recently discovered object larger
> and more distant from the Sun than Pluto). Pluto and Charon would be
> regarded as a double planet, rather than as a planet and satellite,
> because their center of gravity lies outside of Pluto itself (the only
> such case known in our Solar System.) There is a candidate list of
> additional objects that may be large enough to qualify as planets, subject
> to confirmation by the IAU.
>
> The IAU resolution also recognizes Pluto as the prototype of a new class
> of planetary objects to be known as "plutons." In contrast to the
> classical planets, plutons typically have quite non-circular orbits and
> take more than 200 years to orbit the Sun. With increasingly sensitive and
> broad searches of the outer solar system well underway, it is quite likely
> that additional Pluto-like planets will be discovered.
>
> The Division for Planetary Sciences (DPS) of the American Astronomical
> Society is the world's largest international professional society of
> planetary scientists. The DPS Committee, elected by our membership,
> strongly supports the IAU resolution. It was proposed after two years of
> careful review by an international panel of expert planetary scientists,
> followed by a broadly representative international group of historians,
> writers, and scientists. The new definition is clear and compact, it is
> firmly based on the physical properties of celestial objects themselves,
> and it is applicable to planets found around other stars. It opens the
> possibility for many new Pluto-like planets to be discovered in our Solar
> System.
>
> The proposed definition will be brought to the IAU General Assembly for a
> vote on August 24, 2006. As representatives of an international community
> of planetary scienti
Posted by: vexgizmo Aug 16 2006, 05:58 PM
the other effects.
-Alan
But Vesta likely was once "rounded" by gravity, and then later smashed. In fact, its post-impact shape still may prove to be well-approximated by a hydrostatic figure. Again, I like this definition, but I suspect we will be arguing Vesta and others, and arguably Xena doesn't seem to make the cut until we measure its triaxial shape.
-Bob P.
Posted by: David Aug 16 2006, 06:04 PM
I have some thoughts on Vesta's shape, but having gotten myself unnecessarily exercised on this point in the past, I'm inclined to wait until either Dawn or the next generation of telescopes provides a much better image of Vesta's shape than we currently have. I don't mind waiting several years to have an argument.
Posted by: Phil Stooke Aug 16 2006, 06:08 PM
As a rule of thumb for whether a non-spherical world should be mapped on a conventional map projection or a special one (like my morphographic projections), I have said in the past that a 10 percent difference between max and min radii makes a sensible division. Earth's max and min radii (center of mass to top of lithosphere, Chimborazo to floor of Arctic Ocean) differ by 0.5 percent. But like all these choices it's purely arbitrary, and in a situation where shapes will usually be poorly known this will be a serious problem.
People want definitions to be like fences, but usually they are central conditions with fuzzy edges.
I have no problem calling Ceres a planet, but I'm utterly mystified by the idea that Charon should be one. It's a satellite.
Phil
Posted by: JRehling Aug 16 2006, 06:16 PM
[...]
Posted by: AlexBlackwell Aug 16 2006, 06:17 PM
Phil Plait has an interesting http://www.badastronomy.com/bablog/2006/08/15/congratulations-its-a-planet on it.
Posted by: David Aug 16 2006, 07:17 PM
Is that: if the maximum is 110% of the minimum? Or: if the minimum is 90% of the maximum?
Posted by: AlexBlackwell Aug 16 2006, 07:25 PM
I don't have any problems continually redefining "planet," John, especially in light of our ever expanding knowledge base. In fact, I wouldn't have had any problems in using the sizes of Pluto and Charon, with reasonable error bars, as the "bright line" divider for conferring planetary status on an object orbiting the sun. Moreover, given the inevitability of finding "borderline cases" in the Edgeworth-Kuiper Belt, I could have even learned to live with denying planetary status to objects that fell just under the cutoff in size. There are tradeoffs, to be sure, and as Justice Antonin Scalia noted out the outset of the aforementioned oral argument in Roper v. Simmons:
QUOTESo there's -- there's some minimal level of mental retardation. Right? And isn't that necessarily over-inclusive, just as picking any single age is necessarily over-inclusive?
Posted by: volcanopele Aug 16 2006, 07:47 PM
now that I have heard the rational for the double planet scenario (like Pluto-Charon), I am a bit more confortable with it. Basically, they are using historical prescedent to influence this part of the definition. For example, a binary star system may consist of two vastly different worlds, say a blue giant and a red dwarf. The barycenter is located outside either star, but most importantly, both are considered stars. It isn't one blue giant star, and a red dwarf planet (or some such). They are just two stars in the same system. one scenario contemplated was the possibility of two extrasolar gas giants orbiting around a common center of mass. Are both planets? Is only the largest a planet?
Posted by: JRehling Aug 16 2006, 07:58 PM
[...]
Posted by: David Aug 16 2006, 08:08 PM
I feel some discomfort with the idea of a dark object in the interstellar void being called a planet; both on historical grounds, and because such an object, in permanent darkness, conditions of extreme cold, and not gravitationally bound to another object, couldn't be expected to behave like any of the objects we know as planets.
We don't have names for such objects because they haven't been observed. As hypotheticals, I'd prefer "dark object" or, really, anything other than "planet".
As for the Charon question, why not extend the "planetary" honor to planet-sized satellites as well? Just call them "satellite planets" or the like. There's historical justification for this: the Moon, of course, was once considered a planet, and Galileo, Huygens, and Cassini dubbed their finds "planets" before the term "satellite" came to be generally accepted.
Posted by: Jyril Aug 16 2006, 08:13 PM
The barycenter criterion may become handy in the case of a binary where both similar-sized components would qualify as planets (otherwise it wouldn't be clear which one is the planet and which one is the satellite).
Posted by: jsheff Aug 16 2006, 08:13 PM
I don't have a problem with roundness as a criterion, but:
1) why didn't they define an upper limit as well? The phrase "is not a star ..." is insufficient. Has the word "STAR" been defined somewhere else by the IAU? They could have said the upper limit is when a body is capable of initiating a thermonuclear reaction in its core, or that its 13 Jupiters, or whatever. Just saying "is not a star" is a cop-out.
2) They defined a planet, for better or worse. They should have left it at that! It seems most people are objecting to the "double-planet" phrase; personally I'm confused as to why they felt the need to define a "pluton". There are already definitions for things like ""plutinos", or "cubewanos", or "Kuiper objects" or "extended scattered disk" that are already in use by informal convention, and it's been working fine; there has not been any controversy about that.
3) Er, need I state the obvious? ... a planet is a "celestial" object. What does "celestial" mean? In common usage it is used to distinguish something that's not earthly. So doesn't this definition exclude Earth? (We're back down to 11 planets, LOL!)
- John Sheff
Cambridge, MA
Posted by: AlexBlackwell Aug 16 2006, 08:41 PM
For another take on this issue from the blogosphere, see George Musser's http://blog.sciam.com/index.php?title=and_then_there_were_twelve&more=1&c=1&tb=1&pb=1 at SciAm Observations.
Posted by: Stu Aug 16 2006, 10:07 PM
Just a bit of fun...
http://journals.aol.com/stuartatk/TheVerse/entries/2006/08/16/solar-system-v2.0/619
Posted by: dvandorn Aug 16 2006, 11:15 PM
Many Vexed Experts Make Confusing Judgment So Us Normal People Cry 'XXXX'
The wor(l)d denoted by XXXX has not yet been fully explored.
Incidentally I see that the word 'farce' has just entered this discussion. . . quite so.
OK, I was a touch confused by what they were calling planets -- I was thinking that, past Neptune, you'd have Pluto, Sedna, and Xena. I was still thinking of Charon as a moon of Pluto.
Seems all of our paradigms are resisting this shift, to one degree or another...
But, yeah -- I like your version much better than mine!
-the other Doug
Posted by: dvandorn Aug 16 2006, 11:17 PM
I always thought that the rhinocerous was the horse designed by committee...
-the other Doug
Posted by: AlexBlackwell Aug 16 2006, 11:44 PM
Mike Brown of Caltech http://www.gps.caltech.edu/~mbrown/whatsaplanet/.
Posted by: mcaplinger Aug 17 2006, 12:02 AM
Interesting read, but I was a little surprised by his claim of a large number of KBOs that would be planets by the hydrostatic definition. He seems to feel that anything over 400 km in diameter would meet that criterion (presumably if it were mostly ice.) I really think that having >50 "planets" is simply non-viable, and I'm not sure why he poo-poos the simple idea that anything larger than Pluto would be a planet as so non-scientific. Oh well, I guess this is why I'm not an astronomer.
Posted by: dvandorn Aug 17 2006, 12:16 AM
And in this passage of Mike Brown's comments, I see a rather striking agreement with the points I made yesterday:
-the other Doug
Posted by: volcanopele Aug 17 2006, 12:54 AM
I still don't understand what is so bad about having 53 planets, if that's what it ends up being. Or 100. Or more.
Posted by: AlexBlackwell Aug 17 2006, 12:55 AM
Both http://www.astronomy.com/asy/default.aspx?c=a&id=4459 and http://skytonight.com/news/home/3580231.html weigh in.
Posted by: centsworth_II Aug 17 2006, 01:35 AM
It crushes the dream that some of us have had of being the human generation that finally sent probes to all the planets. Not important, I know, but it was a neat thought.
Even with Ceres and Charion becoming planets, that dream would still be realized. But with objects beyond Pluto becoming planets, the human race may NEVER manage to visit all the planets. Talk about moving the goal post.... Oh, well.
Posted by: ljk4-1 Aug 17 2006, 01:36 AM
When I was a kid, we only had 9 planets and they were all in a neat line to the right of
the Sun (which was just a big slice of yellow) and we liked it that way!
And Mars had canals (and maybe ancient cities and certainly some simple vegetation), Venus
was a swamp full of dinosaurs and exotic plants, Mercury roasted on one side and froze on the
other all the time, except for this Twilight Zone area on its terminator where some kind of life
could exist. But otherwise it probably looked just like Earth's Moon. You know, with all those
craters that came from volcanic eruptions. Yeah, a couple guys think that meteorites may
have caused all those pits, but how often does a world get hit from space anyway?
The Asteroid Belt had a lot of objects - maybe tens of thousands, even - but they were
all way smaller than any known planets and probably came from a much bigger planet
that exploded ages ago. Don't ask me how it exploded, okay? It just did, 'cause there's
all that debris. I bet you by the year 2000, astronauts will be out there mining all those
big space rocks for our colonies from Venus to Mars!
Jupiter had a really thick atmosphere with an icy middle and a rocky core, and the Great
Red Spot was from a volcano, and its moons had some dark smudges we could see from
Earth telescopes but that was about all. Saturn was the only planet in the whole Solar
System with these amazing rings, and it had this one moon with just enough of an
atmosphere to make its sky a dark blue.
Uranus was tipped on its side and was surely far more interesting visually than Neptune,
which we only knew had two moons (okay, one of them did orbit backwards) and both
undoubtely were aquamarine in color with atmosphere bands just like their bigger brothers.
Pluto. We weren't sure what the heck Pluto was. It sure wasn't a gas giant planet. It
wasn't even as big as Mercury, though at first we thought it was maybe as big as Earth.
But it was definitely bigger than the Moon and was probably all alone out there in the
deep darkness at the end of the Solar System. There was probably one or maybe two
more planets beyond Pluto, probably much bigger and just harder to find due to the
distance and the lack of illumination they received from the faraway Sun.
After that there were comets in crazy orbits WAY out past Pluto, some of them maybe
even halfway to Alpha Centauri. But that was it. And we liked it that way.
Posted by: AlexBlackwell Aug 17 2006, 01:44 AM
See also A.J.S. Rayl's http://www.planetary.org/news/2006/0816_The_IAU_Redefines_Planet__Pluto_is_a.html at TPS.
Posted by: mcaplinger Aug 17 2006, 02:02 AM
I guess this is one of those "if you have to ask, you'll never know" things, Jason. Kids memorize the names of the planets, but if there were 50 of them to memorize, they won't be able to. I don't know the names of all of Jupiter's moons myself (assuming they all have names.)
Posted by: mcaplinger Aug 17 2006, 02:08 AM
I'm a little confused; this article says that Mike Brown was on the committee, but Brown's web site says he isn't an IAU member and can't vote, and other articles on the web quote him as being somewhat critical of the definition. What's the real story?
Posted by: AlexBlackwell Aug 17 2006, 02:09 AM
Heck, even astronomy students have to cling to mnemonics like, for example, the classic "MET DR THIP" for Saturn's large satellites.
Posted by: mcaplinger Aug 17 2006, 02:23 AM
Feel free to impress me with a mnemonic for the moons of Jupiter. There were 62 the last time I checked, but it looks like S/2000 J11 and S/2003 J1 through J23 don't have names yet. I'm having a hard time doing much with MAATIEGCTLHLEIPHAIETCCPKMSCEKOTEHPEASA myself
Posted by: alan Aug 17 2006, 02:49 AM
List of the 53 planets
http://www.gps.caltech.edu/~mbrown/whatsaplanet/howmanplanets.html
Posted by: centsworth_II Aug 17 2006, 02:51 AM
Well, if kids have to learn http://www.fun-with-words.com/longest_place_names.html names....
Posted by: volcanopele Aug 17 2006, 02:57 AM
It helps if you make a song with all the names. I mean they make it really easy since most rhyme. But yes, to be honest, when I was in elementary school, I knew the names of every moon in the solar system...that was only 61 at the time. But just like the growing list of planets, I haven't seen the need to memorize all the little guys being found now. Doesn't mean they aren't moons, just some are important to know and others aren't. Just as I am sure most planets from now on will be.
But seriously, teaching the solar system shouldn't be about memorizing a list of planets. It should be about teaching the amazing diversity of worlds that exist out there. Some are part of that elite club of "planets", some are mere moons, while others are part of that horde of "small solar system bodies".
Posted by: mars loon Aug 17 2006, 03:30 AM
http://www.gps.caltech.edu/~mbrown/whatsaplanet/howmanplanets.html
that 53 planet map from Mike Brown is quite funny and really drives the point home. An arbitrary size limit of 2000 km would eliminate this problem.
personally I'm glad that Pluto has been saved as a planet, agree the UB 313 (Xena) should be a planet and leave Ceres as a dwarf or minor (but fascinating) planet.
Posted by: David Aug 17 2006, 04:49 AM
As far as I know, kids don't memorize the names of planets; they didn't in my day, when I was the only one in fourth grade who could recite them all (as they were then), something which probably endangered my homeroom teacher's fragile health.
So people will now have to have a table to look up all the planets, just as they have to look at a periodic table to look up all the elements. So it goes.
I've never been quite clear on why a taxonomic distinction should be bound by the needs of a bunch of imaginary schoolchildren, but if I were to teach an astronomy lesson to 4th graders under the new regime, I'd say "here are the inner planets, Mercury-Venus-Earth-Mars-Ceres-Jupiter-Saturn-Uranus-Neptune, and here are the outer planets, Pluto-and-a-bunch-of-others".
Posted by: David Aug 17 2006, 05:14 AM
Heck, an arbitrary size limit of 850km diameter would eliminate the problem. Of Brown's 45 KBOs (he seems to have miscounted, by the way: he lists 54, not 53 "planets"), 15 (or one-third) are under 500km; 24 (or more than half) are under 600km; and 36 (or four-fifths) are under 850km. The remaining 9 range from 940km (2002AW197) to 2400km (2003UB313).
By comparison, only 7 objects in the inner Solar system are in the 400-850km range (and actually all are under 600km diameter). These are: satellites Mimas, Enceladus, Miranda, and Proteus; and asteroids Vesta, Pallas, and Hygiea. Of these Mimas, Enceladus, and Miranda are roundish; Proteus, slightly larger than Mimas, is not; while Vesta, Pallas, and Hygiea seem unlikely to meet the test of "hydrostatic balance". I suspect Brown's list is overlong for practical purposes. He is, of course, entitled to use whatever means he thinks fit to educate people about the population of the Kuiper Belt.
However, the KBOs in the 400-850km range fill in the rather large (600-975km) gap in the sizes of inner Solar system bodies, and it would be nice to have a much better idea of what their shapes actually are. If it turns out that they are predominantly round, then presumably Proteus and Vesta are oddballs. But then again, it might turn out that they are overwhelmingly irregular, and it required some unusual development for Mimas, Enceladus, and Miranda to turn out as (relatively) round as they are.
Posted by: JRehling Aug 17 2006, 05:36 AM
[...]
Posted by: MCS Aug 17 2006, 06:15 AM
Personally, I'm not happy with the IAU proposal. I'm in the demote Pluto camp myself, since to me it just seems to be a large KBO. Any definition that includes Pluto risks trivializing the term "planet" too much, I think, unless it's made clear that it's outside the normal definition, and is being grandfathered in. The "hydrostatic equilibrium" criterion isn't as objective as one would like, either, since there's always going to be a lot of argument over whether a particular body is spherical enough. Even getting enough data to determine whether something is in hydrostatic equilibrium isn't trivial, and is harder than determining the diameter/radius. It might not even be the most important distinction. For example, whether or not something is a KBO seems to be a more important distinction than hydrostatic equilibrium, I think.
At this point, it's probably better to call a moratorium on calling new objects planets until we get a better idea of the size distribution and characteristics of TNOs. I'm thinking that Mercury is about the smallest body I'd call a planet, so something around a 2,000 km radius might be about right, but we don't even know if there are any Mercury-sized or larger TNOs out there, so it seems premature to make a decision now. Whatever the case, I have a really hard time with a definition that can potentially include hundreds or even thousands of objects as planets, so I hope the proposal is defeated.
Posted by: mcaplinger Aug 17 2006, 06:15 AM
Do you have any children? Have you been anywhere near an elemetary school since you were a student in one?
Three years ago, my son's entire first grade class made mobiles of the solar system with the sun and the nine planets, and they had pictures of the planets up on the wall in the classroom. I doubt if that's very unusual. Maybe they didn't memorize the names, but I'd bet that most of them did.
Posted by: mchan Aug 17 2006, 07:44 AM
Regarding the naming of planets that are ejected from the solar system: They can still be called planets if the definition were slightly modified so that a planet can orbit a star or stars. A planet in interstellar space would orbit around the stars at the center of the galaxy.
One could then ask what a planet-like object in intergalatic space would be called. I'd say we can defer answering that until such an object is actually found.
Posted by: remcook Aug 17 2006, 09:01 AM
I memorised the names of the planet when I was very little...as well as the flags and capitals of the world. That's exactly what nerdy children do!
about the discussion... I imagine law school being something like this...
Posted by: AlexBlackwell Aug 17 2006, 04:29 PM
As a result of the current discussion, I believe The Atlantic is offering (temporary?) http://www.theatlantic.com/doc/199802/pluto to David Freedman's article "When Is A Planet Not a Planet?", which was published in the February 1998 issue.
Posted by: hendric Aug 17 2006, 04:41 PM
I guess it depends on what age you expect children to memorize the planets. We learned the 50 states and their capitols in school, why not the 53 planets?
Posted by: AlexBlackwell Aug 17 2006, 04:43 PM
You sound like my wife preparing for a cross-examination.
I'm sorry, but "not having any problems with it" is the best I can do. But, since you're trying to pin me down, counselor, I am in the "if-it-ain't-broke-don't-fix-it-crowd." In other words, this current issue seems, at least to me, more like a solution in search of a problem. Having said that, though, I'm not resistant to change, and I'm certainly not going to lose any sleep over it
Posted by: David Aug 17 2006, 05:27 PM
Okay, enough of this. There aren't going to be any "53 planets". This is a number that has been floated by Mike Brown and Mike Brown alone -- doubtless for very good reasons, but his number is erroneous on several grounds.
One, Brown didn't correctly count up the number of objects in his http://www.gps.caltech.edu/~mbrown/whatsaplanet/howmanplanets.html, which contains 54, not 53 objects.
Two, Brown is using a lower bound of 400km, yet he omits Vesta, Pallas, and Hygiea, all of which are over 400km, so he really should be saying "57 planets" (like varieties of Heinz ketchup
).Three, Brown is the only one using a lower bound of 400km, whereas the IAU proposal explicitly states a lower bound of 800km. Whether or not that is a good figure for determining hydrostatic equilibrium, that is the figure that the IAU proposal actually uses. Using Brown's own figures, this should produce a Solar system of no more than 20 planets, with 2 of those being exceedingly marginal (800km and 810km according to Brown).
If this proposal is adopted, my guess is that when the dust settles, measurements are made, and additional KBOs are discovered, we end up with in the range of 25-30 planets and no more.
Posted by: JRehling Aug 17 2006, 05:57 PM
[...]
Posted by: AlexBlackwell Aug 17 2006, 06:11 PM
Coincidentally, an article, "http://www.cnn.com/2006/HEALTH/08/17/how.kids.learn.par/index.html," turned up on CNN.com today. Given some of the discussion here, it's worth a read.
Posted by: AlexBlackwell Aug 17 2006, 06:56 PM
http://www.nytimes.com/2006/08/17/opinion/l17pluto.html are four Letters to the Editor, including one from Jay M. Pasachoff in Prague, in today's issue of The New York Times.
Posted by: JRehling Aug 17 2006, 07:02 PM
[...]
Posted by: tty Aug 17 2006, 07:16 PM
We have the same issues in biology. For example genus, species and every other taxonomic rank are, in the last analysis, arbitrary. And, yes, there are young hotshot biologists who want to scrap these categories entirely as being unscientific - and just go with cladograms (phylogenetic trees) when referring to and defining plants. But the result of such a proposal would be chaos in terms of how professionals would need to refer to plants (would a forester bother to describe an elm as the second distal branch on the third proximal Magnoliid clade, for example?). And I don’t even want to imagine how amateurs and schoolchildren would cope under such as system.
It is true that genera (and higher categories) are somewhat arbitrary human invention. But species are not arbitrary. They exist in nature as discrete entities that persist through evolutionary time (of course there are exceptions, as always in nature, apomictic plants for example).
tty
Posted by: Jyril Aug 17 2006, 09:37 PM
If there is not now, there will be. For example, consider how probable is that the largest detached KBO (aka Sedna) happens to be near its perihelion *just now*. Scattered disc no doubt includes giants not yet detected. There are certainly far more, even larger objects out there.
Posted by: Jyril Aug 17 2006, 09:44 PM
Most species are not arbitrary because intermediate individuals have died out. If we knew all living organisms ever lived, it would be impossible to say where one species ends and one begins. In fact, there are many extant species where the border between the species is very fuzzy. For example, two species may be linked by varying subspecies/breeds.
However, biological classification is far from unnecessary--figuring out relationships between different organisms is a very important part of understanding them. The situation is very different from the definition of a planet which is fundamentally arbitrary.
Posted by: alan Aug 18 2006, 12:02 AM
If Vesta were made of ice , it would be a comet.
Posted by: mchan Aug 18 2006, 12:48 AM
The TPS list of 7 members of the Planet Definition Committee does not match the member list from the IAU release --
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_committee.html
Specifically, the IAU list does not include Mike Brown as a member. Indeed, the IAU list makes substitutions for two of the member names. Since Mike Brown himself states he isn't an IAU member, I would submit that the TPS list is inaccurate.
Besides, it seems to me there would have been a conflict of interest if Mike Brown had actually been on the committee.
Posted by: slinted Aug 18 2006, 12:55 AM
tty
It's not a bad comparison, as both classification systems are easier to apply for the large bodies (Mammals, birds, reptiles / the original 8 planets) and become arbitrary, strange, and contradictory for the smallest (virus and bacterial species are completely arbitrary, as would the new limitations on KBOs and main belt objects)
Posted by: ups Aug 18 2006, 02:11 AM
Twenty or thirty years from now this debate will fire up again as we discover large bodies in the Oort Cloud.
Posted by: Rob Pinnegar Aug 18 2006, 03:40 AM
Going back to yesterday for the moment:
which means Charon is technically in orbit about the Sun, not Pluto. This is not a debatable
fact, it's just the way the dynamics works.
Hope you'll forgive me for this, Alan, but I can't resist playing devil's advocate: wouldn't that "technically" make Nix and Hydra numberable KBOs, rather than satellites of Pluto?
No wonder the IAU took so long getting this proposal together. No matter which argument you espouse, there's always an immediate counter-argument just waiting to pounce. Also, it's all too likely that the majority will be dissatisfied with ANY solution, no matter what it is. It's pretty much a classic dilemma.
Posted by: SirBruce Aug 18 2006, 06:59 AM
The IAU proposal is simply a bad one. Are you telling me if we found a massive rock the size of Earth, with an atmosphere, but by a recent cosmic collision it was no longer "round", we wouldn't call it a planet? Don't be stupid. The right thing to do is simply assign a size limit, and require that it orbit about a star. The limit should be low enough to include Pluto; whether or not it includes anything else depends on the value agreed upon.
In any case, the IAU proposal contains a fatal flaw. It argues that Charon is a planet because the barycenter of its orbit is outside of Pluto, and thus not orbiting Pluto. Well, guess what? The barycenter of Jupiter's orbit is outside the Sun, therefore it's not orbiting the Sun. Therefore, Jupiter is not a planet under the IAU definition.
I guess they didn't think about that.
Bruce
Posted by: mchan Aug 18 2006, 08:07 AM
I think the proposal is clever in that its effects are spread out and changing over time.
The proposal neatly divides gravitationally rounded bodies into 3 groups --
1. "classical planets" -- the 4 terrestrial and 4 gas giants
2. "dwarf planets" -- Ceres and a few asteroid candidates
3. "pluton planets" -- Pluto, Charon, 2003UB313, and about 10 more TNO candidates
Initially, the public attention will be is on "planets" without any modifiers. There had been only 9 planets in the public's view for over 75 years. The sudden change from 9 to 12 garners heavy media focus on planets, e.g., "oh look! 3 new planets!" Pluto stays as a planet, and the IAU avoids immediate public disapproval associated with taking away the "planet" label from Pluto.
Over the next few years, additional TNO's and possibly asteroids will be officially designated as planets. The IAU can manage this slowly, e.g., the next biggest TNO becomes a planet in a few months with one or two more TNO's making transition every one or two months. The names will be mentioned and then forgotten by the most of the public.
In the case of the first additional "pluton planet", media attention may focus on planet, but astronomers can begin to publicize that the TNO is a "pluton planet" or a "pluton" in the shortened form. Likewise, if Vesta or Pallas is re-categorized as a planet, the publicity will shift to emphasize "dwarf planet".
As additional TNO's become planets, media attention will dwindle. What attention there will be will note the "pluton" descriptor more strongly. Astronomers will also emphasize that Pluto is a "pluton". See here, Pluto sounds more like pluton than planet! The names of the new plutons will be even more quickly forgotten by the general public. The public will come to know that there are a lot of "plutons" out there.
As time passes, the number of "pluton planets" (with the emphasis on "pluton") will continue to increase, but the number of "classical planets" will remain at 8. In my observation, most people with a passing interest in astronomy prefer simple explanations that don't keep changing. The general public view may go back to remembering that for over 75 years, there were only 9 "planets". However, over time, the public will also become accustomed to occassionally hearing about new "plutons", and that Pluto is a pluton.
Astronomy books and websites will categorize the "planets" into the three groups above. School children will learn the names of the 8 "classical planets" and will know Pluto as the first of the "pluton planets". The names of other plutons will only be known as answers to bonus test questions.
The public view may evolve to the view of some earlier astronomer proposals regarding Pluto's status. E.g., there will be a lot of "planets" in the scientific use of the term, but the public use of the term will revert to 8 or 9 planets with the 9th being also recognized as a "pluton". Since Pluto will have a special status as the genesis of the name of "plutons", the other 8 "classical planets" will also have a status that they are not "plutons". See here, the names of the 8 "planets" do not sound anything like "plutons". The public view may evolve further that there are only 8 "planets" that have names that school children memorized, with the 9th being Pluto the most famous of the "plutons".
The situation with Ceres and any additional asteroids that become planets may be analogous. Initially, the planethood of Ceres will be celebrated. When Dawn reaches Ceres (and possibly Vesta earlier), astronomers may choose to refer to them as "planets" but will emphasize that the "dwarf" modifier. Again, they will be distinguised from the 8 "classical planets".
Over time, the scientific use of "planet" will diverge from the general public use of "planet". The general public will associate "planet" to the 8 "classical planets" plus the well known Pluto out of the tens or hundreds of "plutons" whose names will not be remembered. Pluto will be an honorary planet in the public view.
Posted by: David Aug 18 2006, 12:48 PM
My guess is that any collision with enough energy to visibly deform an Earth-sized rocky planet would also produce enough heat to restore it to a molten state; which, on cooling, would revert to a spherical shape. This is not entirely hypothetical, as a collision of this nature has happened once already in the early history of the Solar system; the result was not jagged, irregular Earth-sized fragments, but the Earth and the Moon -- both of which are round.
Posted by: djellison Aug 18 2006, 01:06 PM
Here's a question. What is 'round'. We need to have that defined. The Radius must vary from the mean by no more than 10% say. Something like that.
Doug
Posted by: David Aug 18 2006, 01:43 PM
Doug
Here are some measures of oblate flattening of some Solar system objects (taking the longest and shortest axes as a basis for measurement):
2003 EL61: 0.492
Vesta: 0.208
Saturn: 0.098
Mimas: 0.081
Proteus: 0.078
Ceres: 0.068
Jupiter: 0.065
Iapetus: 0.049
Enceladus: 0.032
Uranus: 0.023
Neptune: 0.017
Mars: 0.008
Earth: 0.006
Moon: 0.003
Once you get below 0.04 the difference between a spheroid and a sphere becomes pretty hard to judge with the naked eye, except under very favorable conditions (exact alignment of the short and long axes, full view of the illuminated hemisphere).
Proteus is a curious beast, because although its dimensions are within 10% of each other, it presents a decidedly non-round appearance, showing from various vantagepoints the silhouette of a rectangle or pentagon rather than a circle or ellipse.
Posted by: ngunn Aug 18 2006, 01:44 PM
Doug
As someone else pointed out further back, 'round' in the new IAU lexicon seems to mean not 'sperical' but 'in hydrostatic equilibrium'; i. e. rotational flattening, or in extreme cases rugby balls - possibly even dumbells - are OK, as are tidal bulges raised by close companions. Not sure about fossil tidal bulges though.
While we're at it here's another awkward case: A three body system has it's barycentre inside the primary when the two companions are on opposite sides but in free space when they're on the same side. Question: how many of the three could be planets? To make this sort of nightmare go away all they have to do is replace the barycentre condition with a simple mass-ratio-to-primary one. After all, the masses in multiple systems are usually easier to determine than the exact shape and size of each object.
Posted by: Jyril Aug 18 2006, 01:58 PM
It would be strange if he was a member, because he is clearly http://www.gps.caltech.edu/~mbrown/whatsaplanet/.
Posted by: Jyril Aug 18 2006, 02:09 PM
Zillionth time: The proposal is poor, and so is any other because there exists no good definition. "Planet" is fundamentally cultural term, wanted we that or not.
Let's hope they vote for it so that we finally can stop that eternal dispute.
In the end, this may turn out positive for us planetary probe fanatics. If an object receives a planetary status, it gets more PR and is more likely visited by a probe (at least in the main asteroid belt). Ceres was badly outlooked because it was considered just another "rock" not a Real Planet™.
Posted by: tasp Aug 18 2006, 02:15 PM
From Rob:
Hope you'll forgive me for this, Alan, but I can't resist playing devil's advocate: wouldn't that "technically" make Nix and Hydra numberable KBOs, rather than satellites of Pluto?
My comment:
The deflection of the Pluto/Charon barycenter by Nix and Hydra being less than the radius of either Pluto or Charon should secure these two bodies as satellites.
Posted by: ngunn Aug 18 2006, 02:36 PM
Exactly, so why bother? For PR purposes? Is that really what the IAU is there for? I'm sad and surprised to think that worlds like Ganymede or Titan might be considered less worthy of exploration because they each orbit a planet as well as the Sun. Why should that matter? It must be so because even the Cassini team seems to think so - the preamble to each Titan flyby page starts with a statement to that effect. I'm totally mystified by this.
Posted by: ljk4-1 Aug 18 2006, 03:53 PM
According to Daniel Fischer from the HASTRO list, I quote:
There has just been the first open debate here at the IAU General
Assembly on the proposed resolution in which only planetary
scientists (planets of all sizes) took part: The proposal lost,
about 60:40, to an alternative put forward by a group of other
planetologists (which would have made "being by far the largest
object in the local population" plus roundness the criteria for
being a planet and thusly excluding Pluto). The term "plutons"
was rejected in a 2nd vote by an overwhelming majority, for
the linguistical confusion it may trigger (and has already done
so in places). None of these votes is binding in any way:
It's up to decide for the IAU Executive now to decide on
further action ...
Daniel
Daniel F. just posted another note with a link to the details, plus this:
More on today's events: http://www.astro.uni-bonn.de/~dfischer/mirror/300.html
I'm now being told that the vote was actually more like 75:25; the counting
was a bit chaotic ...
Daniel
Posted by: David Aug 18 2006, 04:39 PM
about 60:40, to an alternative put forward by a group of other
planetologists (which would have made "being by far the largest
object in the local population" plus roundness the criteria for
being a planet and thusly excluding Pluto).
Groan. Okay, let complete chaos ensue... cry havoc, and let slip the dogs of war.
I don't find the alternative proposal any clearer than the primary proposal. How does one measure a "by far"? What constitutes or does not constitute a "local population"? In what system of analysis do all TNOs constitute a single "local population"?
1 Ceres is three and a half times more massive than 4 Vesta. Is 3.5 too small for "by far"?
Sedna isn't even a KBO by the most generous definitions of the Kuiper Belt. Does that make it "by far the largest object in [its] local population"?
2003 UB313 also has an orbit that takes it well beyond the outer edge of the Kuiper belt. Strictly within the KB itself, Pluto is still the largest object discovered. But "by far"? Who knows what that means?
If the KB constitutes a single "local population", then perhaps the much smaller region inside Jupiter's orbit can also be considered a single "local population". The largest objects in this region (Earth, Venus) are close in size -- and both are much smaller than Jupiter -- so neither of them can be considered a planet!
Posted by: JRehling Aug 18 2006, 05:50 PM
[...]
Posted by: ljk4-1 Aug 18 2006, 06:10 PM
What is a planet? by Steven Soter
http://arxiv.org/abs/astro-ph/0608359
Posted by: mcaplinger Aug 18 2006, 06:42 PM
The question of what a planet is has one important ramification: it affects what the name of 2003 UB313 can be. I think we're all tired of saying 2003 UB313, so it'd be nice if the IAU could resolve the question to that level.
Posted by: David Aug 18 2006, 06:50 PM
I agree. However, there is an easy way for the IAU to resolve the question without making a decision on "what is a planet" -- let them abolish the rule providing for a different system of nomenclature for planets.
Posted by: volcanopele Aug 18 2006, 06:58 PM
all sun-orbiting objects get numbers.
Welcome to 145675 Earth.
Posted by: Planet X Aug 18 2006, 07:08 PM
Welcome to 145675 Earth.
Hehe, good one! Don't forget to visit 145676 Luna (Earth's Moon) while you're at it! Later!
J P
Posted by: JamesFox Aug 18 2006, 07:10 PM
http://arxiv.org/abs/astro-ph/0608359
I've just read this proposal. It completely ignores the actual physical attributes in favor of pure orbital population dominance. This position is pretty diametrically opposed to Alan Stern's. It does contain some questionable assumptions, like the 100:1 miniumum ratio for object-mass:mass-of-everything-else for planet qualification, and the fact that he says every single KBO can collide with Pluto.
Also, it uses accretion terminology in the definition. He seems to feel that intermediate cases when it comes to population dominance are impossible due to accretion mechanics.
Posted by: David Aug 18 2006, 07:13 PM
I was amazed by Soter's comment myself:
My initial comment is that of Spock on Khan:
Here's another surprising one from Soter:
So he transitions from "more massive than the total mass of all other bodies" to comparing the mass of one object to the next most massive body, and likewise transitions from "similar orbit" to "cross[ing] its orbit". I think there may also be another body in the neighborhood of Earth with an orbit that frequently crosses Earth's that's just a bit bigger than 1036 Ganymed.
Posted by: JRehling Aug 18 2006, 07:39 PM
[...]
Posted by: JamesFox Aug 18 2006, 07:47 PM
I think he overlooked that possibility when he excluded objects in resonance with each other from counting when it comes to crossing orbits (when discussing one of the extrasolar planet systems). He also exludes resonance with Neptune when discussing objects crossing Neptunes orbit, but ignores that this implies some KBO's have orbits synced with each other.
Personally, I think it might be interesting if some sort of hackish compromise between this and the IAU proposal could be hammered out that might be not too disgusting to either the 'type of object' or the 'population dominance' crowds, by making a more explicit distinction between major and dwarf planets.
Posted by: alan Aug 18 2006, 08:10 PM
Pluto crosses the orbit of Neptune, but its 3:2 mean motion resonance with the planet shields it from a collision. However, all known KBOs cross the orbit of Pluto and can potentially collide with it.
Their orbits may not intersect with Pluto's now but orbits do precess. Eventually this potentially brings them all into orbits which could collide with Pluto. Using mass ratios will tend to break down eventually as border line case may be found in other sysstems. I guess he uses them because he needs a way to quantify "a planet is large enough to dominate its orbital region"
I think a better way to state this would be a "planet is large enough to remove, absorb or control other objects in its region during a the life of its system"
Posted by: JRehling Aug 18 2006, 09:20 PM
[...]
Posted by: SethCohen Aug 18 2006, 09:30 PM
Yes, it's a term that's more cultural in origin than anything else, but I do I think SOME definition has to be agreed upon, because in addition to its use by astronomers when talking to the public about our solar system for years, modern astronomers have made it very clear to the public that they've found other PLANETS around other stars. That implies that there is some important, universal meaning of the word planet. It's time for "planet" to be defined.
Every field has its taxonomy and nomenclature and sometimes the divisions between kinds of objects are arbritary, and in areas as old as astronomy (and with as much amateur influence) a fair amount of terms won't be what one would want if he/she were working from scratch. We do have other choices besides these proposals, like simply going absurdly generic and totally unscientific and calling almost anything that goes around a star a planet, but that makes astronomy look even worse to the general public who are part of the reason why this debate is being held in the first place. (And public perception does matter when it comes to research and exploration.)
In any event, I'm probably the only one who thinks the "first" main proposal is about as good as we're ever going to get.
Posted by: Rob Pinnegar Aug 18 2006, 09:43 PM
How about we avoid the whole mess and have a different category for every object? Thus Earth could be the only "Earth", Venus the only "Venus", and so forth.
When we run out of names, we can start on numbers. Those don't run out as quick.
Posted by: JRehling Aug 18 2006, 10:47 PM
[...]
Posted by: David Aug 18 2006, 10:52 PM
I went back to some reference works from 1930 (the year Pluto was discovered) to try to figure out what people 76 years ago would have thought of the controversy.
The first thing I found was that back then the word "planet" was inclusive in meaning, not exclusive: any non-luminous body revolving around a star was a "planet". The question at the time was not how to define "planet", but how to define "minor planet" -- then the technical term for "asteroid".
Minor planets were distinguished from planets by two factors: first, being smaller in size; second, being numerous and (mostly) confined to the area between Mars and Jupiter. To some extent the latter distinction is haunted by "Bode's Law"; it was believed that there was room for one planet between Mars and Jupiter, but to find several implied that they were all something less than a planet.
The identification of Pluto as a planet follows directly from these ideas. It's true that Pluto was once thought to be about the size of Earth, but even if its true size had been known, I think it would still have been identified as a planet. The Kuiper Belt was quite unknown then; Pluto wasn't inside Jupiter's orbit; it wasn't a sungrazer like a comet; and it was out in that vasty unknown where "Planet X" had been postulated. Hence, it had to be a planet.
If a large population of KBOs had all been discovered at the same time, it's anyone's guess what would have been done. The classification of Pluto as a planet has been considered a "mistake", but in light of the absence of evidence that it was one of a swarm, there was really no other alternative.
Since there seems to be no consensus on what a "planet" is, or any agreement that "major planets" constitute a single group in anything but the negative, perhaps it is better to ask the reverse question: what is a "minor planet"? If both asteroids and KBOs belong to the latter group, what unites them? If we have a unified, positive answer to that question, then the definition of "major planet" can be made in purely negative terms; it is a planet that isn't a minor planet. There need be no question of demotions -- because every object but the Sun would be in some sense "a planet" -- but just a question of whether something fits the minor planet category or not.
Posted by: JamesFox Aug 18 2006, 10:56 PM
http://arxiv.org/abs/astro-ph/0608359
Just one other thing: it occurs to me that the definition presented makes it impossible to define the planets in a system until it has completely finished formation and settled into a steady state, which may take hundreds of millions of years.
Posted by: ElkGroveDan Aug 18 2006, 11:03 PM
I haven't been following this debate because to me it's a dumb semantic exercise.
But if no one has suggested it yet, why don't we consider Bode's Law within a defined range? That way we get to keep Pluto (more or less), Ceres is in, but Neptune is out.
It's as valid and as arbitrary as any other of the many sets of rules tossed about this week.
Posted by: JamesFox Aug 18 2006, 11:05 PM
The first thing I found was that back then the word "planet" was inclusive in meaning, not exclusive: any non-luminous body revolving around a star was a "planet". The question at the time was not how to define "planet", but how to define "minor planet" -- then the technical term for "asteroid".
....
Since there seems to be no consensus on what a "planet" is, or any agreement that "major planets" constitute a single group in anything but the negative, perhaps it is better to ask the reverse question: what is a "minor planet"? If both asteroids and KBOs belong to the latter group, what unites them? If we have a unified, positive answer to that question, then the definition of "major planet" can be made in purely negative terms; it is a planet that isn't a minor planet. There need be no question of demotions -- because every object but the Sun would be in some sense "a planet" -- but just a question of whether something fits the minor planet category or not.
Interestingly, I've tried to bring up the 'minor planet' issue before, and it seems that perhaps the name 'minor planet' is no longer in favour: the only reply I got insisted that 'minor planet' does not imply the object is a planet, just in the same way some astronomers seem to think 'dwarf planet' is not a real planet either. Every single definition or discussion of 'planet' I've seen so far ignores the term 'minor planet'. The IAU proposal explicitly drops the term.
Personally, I think that perhaps one could define planet the old way, and then divide them up into minor (non-spherical), major (the most important ones), and meso (everything else), similar to the Asimov proposal. The meso-planet category is pretty much exactly like the 'dwarf planet' one, except that the term acknowledges the term 'minor planet'
Posted by: David Aug 18 2006, 11:08 PM
I'm not sure that's quite an apt comparison; individual names can be quite eccentric, but they're generally not good clues as to the class they belong to. New York City is a city, but Pentagon City is an unincorporated neighborhood. Charles Town, on the other hand, is a city. Collier Mountain, whatever its name, may in fact be a hill. Attempts have been made to define "hill" and "mountain" (the latter sometimes being said to be a rise of at least 2000 feet above surrounding terrain); but these definitions are necessarily arbitrary, as there is a pretty complete spectrum of peak heights.
Because of the relatively small number of large objects in the Solar system, there are necessarily going to be gaps in the sizes (and other characteristics) of those objects rather than a continuum; and there's a tendency to hope that such gaps are natural rather than adventitious. The continuiing ability of discovered extrasolar planets to break every rule of thumb we thought we knew about planetary distributions suggests to me that this is a forlorn hope, however.
Posted by: DonPMitchell Aug 18 2006, 11:24 PM
Whatever happened to the word "planetoid"?
And if a planetoid hits the earth someday, will it be a planetite?
Posted by: JRehling Aug 18 2006, 11:25 PM
[...]
Posted by: ljk4-1 Aug 19 2006, 02:21 AM
To add some needed humor to this debate, Stephen Colbert takes on Neil
deGrasse Tyson for his removal of Pluto from the Hayden Planetarium of
the main planet members.
http://www.youtube.com/watch?v=b-7l2G2a6js
And maybe this is coincidence - or not - a recent New York Times article
discusses the mathematical concept that anything can be a sphere so long
as it doesn't have a hole:
http://www.nytimes.com/2006/08/15/science/15math.html?_r=1&ei=5087%0A&en=f7174b7c2ac85627&ex=1155873600&pagewanted=all&oref=slogin
Shudda just called everything either planetoids or Kuiperoids.
Tombaughoids?
Posted by: Ian R Aug 19 2006, 08:34 AM
And of course, Uranus must be reclassified as a Hemorrhoid...
Posted by: MCS Aug 19 2006, 09:26 AM
I guess what unites asteroids and KBOs is that they're swarms of objects. While the largest members are noteworthy, for the most part they're large groupings of indistinct objects. The classical planets are unique enough to not be part of a larger swarm. There are subgroupings, but not large, uncountable swarms.
One can argue whether the largest asteroids, KBOs, and whatever else are distinct enough to be major planets. Is Pluto just a large KBO, or does it stand out enough to be classified as a relatively unique object? Ceres is possibly more distinct from other asteroids than Pluto is from other KBOs, so is it just an asteroid, or something more?
If it's at all important to have a formal definition of what a planet is, and I'm not convinced that it is (though it has generated a lot of discussion!), then I think it should be a definition that's only going to include a small number of the largest, most distinctive, most unique bodies in a system. I think we should wait until we have better knowledge of the characteristics of the TNO population before settling on a definition, though. The current proposal could allow for hundreds of planets, which seems too much like a swarm of objects to me.
Posted by: Stu Aug 19 2006, 10:47 AM
An alternative proposal now means Pluto might be demoted after all, and the number of planets in our solar system not increased but reduced, to just eight...
This is starting to sound a bit like a Monty Python sketch, dontcha think? I can see the vote next Thursday... an astronomer staggers into the hall carrying an enormous spherical object. Another shouts over at him...
"What's that you've got there?"
"Pluto. It's a planet..."
"No it's not - it's a dwarf".
"A dwarf? Nonsense. You can clearly see it's a planet..."
"I can see no such thing. It's too small. Weird orbit, too. No, it's a dwarf, or a Pluton. Either way it's an ex-planet. A deceased planet. A planet that never was in the first place - "
"A Pluton? Are you out of your bleeding mind? LOOK at it!" (astronomer bangs planet on table to emphasise its mass) "What about all those innocent little kiddies! Are you going to be the one to tell them their rhyme is wrong? It's a planet!"
"Not according to my list here..."
"Right! If it's NOT a planet then what is it?"
"Calm down! I didn't expect the Spanish Inquisition - "
(Figures in red burst in from side door)
"Ha! NO-ONE expects the Spanish Inquisition...!!!"
Posted by: dvandorn Aug 19 2006, 04:03 PM
"This *is* the IAU, is it not?"
"Yes, sir, it is."
"And... you really don't have any definitions of what's a planet, do you?"
"Ummm... no, sir. We don't. Sorry."
"I see. You do realize, I'm going to have to shoot you, now."
-the other Doug
Posted by: vexgizmo Aug 19 2006, 05:55 PM
Wow--backlash!
http://www.space.com/scienceastronomy/060819_new_proposal.html
Apparently Charon-as-planet crossed the line.
Perhaps Christine Lavin was prescient:
"Scorpios look up in dismay
because Pluto rules their sign;
Is now reading their daily Horoscope
just a futile waste of time?"
Posted by: volcanopele Aug 19 2006, 06:45 PM
hmm, so what exactly does "by far the largest object in its local population" mean exactly? How is that defined? Without a precise understanding of what that means, you could expand or shrink the region to what ever fits your fancy. Titan is the largest object by far in its local population, is big enough to be roundish, isn't a star. Wow, it must be a planet. Yeah, this one is going to open up a whole new can of worms.
Posted by: Stu Aug 19 2006, 06:52 PM
I would have thought "Titan's local population" would be Saturn and the rest of its moons... so Saturn would be the largest object in that population..?
Posted by: David Aug 19 2006, 07:41 PM
Or, as I said, chaos.
Here's how they define "local population":
[1] The local population is the collection of objects that cross or close approach the orbit of the body in consideration.
I suspect that, historically, this notion of planetary uniqueness in a "locality" has something to do with the Ptolemaic conception of the crystalline spheres in which planets were embedded; these spheres had to be nested within each other because obviously they couldn't interpenetrate.
I don't know what a "close approach" is technically, but I suspect that Sedna doesn't "closely approach" any other object currently known. It isn't just the largest object in its local population; it is (as far as we know) currently the only body in its local population. As we detect more objects deeper into space, that is likely to change, but that points up a problem in the definition: it allows objects to be classified as planets on first discovery (if applied retroactively to Ceres in 1801, it would have unambiguously allowed Ceres to be classified as a planet -- as of course it was) but then allows the objects' planetary status to be yanked if it turns out there are more of the same kind.
In terms of general classification this is not a problem, but if planetary status is linked to nomenclature it is a problem, because it means that newly discovered objects may be stuck with names that are (retroactively) inappropriate. As may become the case with (UNNUMBERED) Pluto. It would be nice to have a way of fixing the identity of an object soon after discovery in a way that could not be altered by subsequent discoveries. It would also be nice to not upset current classifications too much, at least not ones that have been in place for decades or centuries. For this reason I would have supported a thoroughly arbitrary 2000km diameter or 10^22kg mass boundary, either of which would have retained current classifications untouched. But this kind of thinking has been dismissed as "unscientific".
Posted by: volcanopele Aug 19 2006, 08:21 PM
But it depends on how you define "local population". I could define it as those objects that orbit in the outer part of the Saturn system. In which case, Titan is the largest object. the definition makes no stipulation that a "planet" must orbit a star. This would thus open the door for at least 16 objects (plus 1-3 borderline cases) currently classified as "moons".
Posted by: djellison Aug 19 2006, 08:25 PM
I think it actually does.
"A Planet is a celestial body that (a) has sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and ( b ) is in orbit around a star and is neither a star nor the satellite of a planet."
Doug
Posted by: volcanopele Aug 19 2006, 08:28 PM
In the official proposal, yes it does stipulate that. But in the alternative definition linked to by vexgizmo, there is no such stipulation:
Thanks to that little omission, I would support this. I'd take the new planets Io, Titan, Enceladus, Europa (yes, even Europa), and Triton over Pluto ;)
Posted by: alan Aug 19 2006, 09:25 PM
Maybe they should use Justice Potter Stewart's definition "I know it when I see it"
Posted by: David Aug 19 2006, 09:33 PM
Ironically, the satellites of our Solar system are (probably adventitiously) far more amenable to taxonomic classification than the planetary bodies. They can be split into three categories, which might be called Major Moons, Mesomoons, and Minor moons, or more whimsically Satellites, Selenoids, and Moonlets.
Category 1 is a set of objects that cluster around or just below the lower end of what are currently called "planets": the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton -- these are all large and, for the most part, interesting bodies.
Category 2 is a set including objects smaller than this (after a quite large gap) down to the minimum "round" sizes: Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, Miranda, Ariel, Umbriel, Titania, Oberon, Charon -- these are all icy satellites which -- with the notable exception of Enceladus -- are mostly interesting for their craters, though Iapetus and Miranda have pretty interesting topography.
Category 3: everything smaller.
I realize this is all after-the-fact justification on my part, and quite useless to inject into the controversy at this point, but what about "large enough to retain an atmosphere" (whether it actually has one or not) as a pseudo-natural basis for a dividing line? Since we seem to have established that Enceladus' atmosphere is really a localized mist due to the southern polar geysers, that pretty much lets off anything smaller than Pluto.
Posted by: vexgizmo Aug 19 2006, 09:58 PM
Category 3: everything smaller.
Poor Proteus; bigger than Mimas, but forgotten again.
Posted by: volcanopele Aug 19 2006, 10:00 PM
Well, that's what it gets for being a slacker. Not having the common decency of pulling itself into a roundish shape. If it did, people wouldn't forget it.
Posted by: DonPMitchell Aug 19 2006, 10:50 PM
Aren't a lot of the asteroids now believed to be "rubble piles"? Seems like there is a difference between a solid body that is sphereical, and a pile of boulders that might settle into a sphereical shape.
Posted by: Jeff7 Aug 20 2006, 03:08 AM
Just chiming in, I'm in the camp that says Pluto is a Kupier Belt object.
I saw it once suggested to create a separate class: Classic Planets. That would include the 9 we now know. Pluto would have a side note, in that it is likely a Kupier Belt object, but was mistaken as a planet due to the comparatively inferior technology at the time of its discovery.
But after that, well, that's about it. Anything beyond the orbit of Neptune seems like it should be classified as a Kupier Belt Object.
But who knows, in a few hundred years, when we discover an object made/coated entirely of whatever covers the dark side of Iapetus, is the size of Titan, and is maybe 75AU from the sun, and has a regular orbit, maybe then we'll have this same argument all over again.
It seems space just keeps getting weirder and weirder, constantly challenging our terminology and classification systems. Even here on Earth, new animals are still being discovered which defy classification.
Simply getting the public to know about the 9 classic planets seems enough. Maybe add in that there are other sizable chunks of matter way the heck out there, but if they could just know that Mercury, Venus, Earth, and Mars are the rocky ones, while Jupiter and Saturn are almost entirely gas, and Uranus and Neptune are icy rocks with thick, dense atmospheres, with little Pluto on the outskirts, well, that's probably more than most know right now. Save the 53 planets, or however many it may be, for the more intensive science/astronomy classes. Otherwise, people will hear "53 Planets" and just figure "Oh well, it's too complicated for me to bother with at all."
Interesting sidenote on the definition of a planet - the object must be "round". How about Saturn? From NASA's website:
"The planet's diameter is 13,000 kilometers miles (8,000 miles) larger at the equator than between the poles."
Earth's diameter is right around that much. How round is "round?"
(I'm not seriously debating Saturn's planetness, just pointing out how abstract wording might cause more problems.)
Posted by: MCS Aug 20 2006, 05:09 AM
This is why I think it's premature to settle on a definition for planets. I think we need more time to find out what the distribution of TNOs is like.
I think arbitrary cutoffs are as good as anything at this point as well, though I'd set the limit higher, at a 4,000 km diameter, or a mass of 10^23 kg. That would put the cutoff at Mercury. Not that I dislike Pluto (I have my name going there and beyond on New Horizons), but I can't help seeing it as more of a KBO than a planet.
I do like the rationale of the alternative proposal that a planet should be an object that was a dominant body in an accretion zone. That satisfies my objection to definitions of planets that could include members of swarms of objects. Maybe one could base a cutoff on the magnitude of error in the ephemerides of nearby objects if the object in question is excluded.
The problem of getting planet status retroactively yanked seems to be a problem with any definition as long as the information used to classify an object is uncertain and incomplete. That's a fundamental problem with Pluto - it's much smaller than it was thought to be, and it's part of a class of objects that weren't known of at the time it was discovered and labeled a planet.
Posted by: volcanopele Aug 20 2006, 06:33 AM
This will only work until we find a KBO larger than Mercury...then the real fun begins.
Posted by: Stu Aug 20 2006, 06:33 AM
I'm actually in the "Save Pluto" camp, not just because it's big enough by some of the standards being ping-ponged about but simply because - and this makes me something of a Johnny No Mates, I know - of tradition and history. I know, they're "dirty words" to some people who live their lives in the sterilised, digital scientific world and have no time for such concepts, and my reasoning isn't exactly scientific I know, but we've all grown up with it as a planet, most of the men and women in the street think of it as a planet, and... oh, I don't know... in my gut it just feels wrong and hideously disrespectful to Clyde Tombaugh to demote Pluto just on a scientific principle, you know?
I mean, don't forget, 70 years ago, before the luxury of automated searches with computerised telescopes, in the days when people actually took those great fat photographic plates of the sky, the guy laboured for what must have been hundreds of hours, sweating over those plates in his clunky cyberpunk blink comparator, with absolute dedication. No lazy digital surveys or computer programs in those days... hunting was done with the good ol' Mark One Eyeball!. He found Pluto basically manually, extending the solar system by a magnitude, and we're going to sweep that away just because it's inconvenient for us and possibly confusing for the astronomers of the future? I just feel uneasy about that.
I'm absolutely sure there's a good scientific case - with many points - against Pluto retaining its planetary status. Many very knowledgeable people have made some great arguments for that here, but in my mind this issue is just as much about personal respect as it is about astronomical facts and figures. Just leaving Pluto alone, giving it unspoken honourary planet status, would hurt no-one, would it? I mean, we'd all know it's an anomaly, but we'd just acknowledge it sneaked into the planetary party without a genuine invite but now it's in just Let It Be. Would that really be so awful? In doing so would not be dismissing or disrespecting the work of a wonderful observer who gave the world a strange a distant planetary outpost on the frontier of the Deep Dark. And with Pluto sitting happily in the corner, content to just sip its drink and not make any trouble, we could draw up new entry rules for the planetary candidates we now all know the names of, unhindered by preconceptions about Pluto. ( But maybe that's just me being naive and romantic, and I'm sure some people here will have a little sneer at that. But hey, I can live with that. )
To many people "out there", the oft-quoted "men and women in the street" this seems very petty and pointless, astronomers tinkering with things just for the sake of it. I know this because I have had conversations with many of them about it, at work and literally in that street. Keeping Pluto a planet just because it's thought of as a planet isn't a good enough reason in itself, I know, but we should have more respect for that view because if we don't we're - that's the astronomical community, the people who "do" and care about this stuff every day - going to come across as rather pompous and uncaring, I fear.
If it was decided to keep Pluto as a planet, then admitting that part of the reason for that was honouring the planet's history and the common man's conception of it would go down very well, I think, make us seem more human at a time when scientists and those who have a passion for science are not viewed very generously "out there".
We really could just leave Pluto sat over there in the corner enjoying the music. Besides, for purely selfish reasons, I want Pluto to remain a planet because I haven't seen it yet, and when I catch my first glimpse of it I want to be looking at a planet, not a "Pluton" or an "icy dwarf" or whatever.
Posted by: volcanopele Aug 20 2006, 06:42 AM
Nice argument. I would definitely consider myself in the "Keep Pluto as a Planet" crowd. When objects like Jupiter and Earth are lumped in the same category, the word "planet" has long sense left behind any real use for myself at the very least, so I don't see the harm in keeping Pluto as a planet.
Posted by: Stu Aug 20 2006, 06:44 AM
Thanks!
Now all we have to do is convince everyone else in the world...
Posted by: David Aug 20 2006, 12:17 PM
Here's another idea:
If we:
1) Admit that the distinction between planet and other solar system object is 'cultural' and not scientific;
2) Think it's very important that "schoolchildren" (or whoever) have a settled, easily graspable number of planets to memorize;
3) ) Don't care that an object might be identified as a planet and then later deplanetized;
Then why not simply declare that a planet is one of the 9 or 10 or 12 largest sun-orbiting bodies in the Solar system, and let the bottom 3 or so cycle in and out as new discoveries are made?
Yes, I know it's a terrible idea. But is it that much worse than some of the other proposals that have been made?
Posted by: Stu Aug 20 2006, 12:38 PM
Hmmm... that could lead to the nightmare scenario of Earth being demoted from planet status if a whole clutch of massive objects was discovered in the Deep Dark. Possible... we only found Sedna cos it was at its orbit's closest point to the Sun... if we found half a dozen Earth-sized bodies out there we'd be out. How would that go down across the world, telling people those big bad astronomers said they don't live on a planet anymore...
Hey, once we get to the 30s or 40s for planets, we could have a "Planet Idol" contest on TV, where each week astronomers have to make the case for their chosen planet. Those that impress Simon Cowell and the viewing voters get to stay as planets, those that don't are rejected and end up serving fries in burger king for the rest of their lives...
Posted by: SFJCody Aug 20 2006, 02:59 PM
Mike Brown's commentary:
http://www.gps.caltech.edu/~mbrown/whatsaplanet/revolt.html
An 'interesting' report from S&T:
http://skytonight.com/news/home/3601616.html
Posted by: volcanopele Aug 20 2006, 05:13 PM
Thanks for the link. Though, "They would quickly come to the conclusion that there are 8 major bodies orbiting the sun. (Well, maybe they would only say four: Jupiter, Saturn, Uranus, Neptune. But I'm not yet ready to lead the fight to demote Mercury, Venus, Earth, and Mars.....)." --- If this proposal does pass, and no one notices that it allows the largest moons to become planets, than I will lead that fight. Hmm, I wonder if www.demoteearth.com is taken...
Posted by: dvandorn Aug 20 2006, 05:48 PM
I like it! And then we have the opportunity to have the following conversation:
"Hey, how many fries do you get in the Cosmic Size?"
"Billions and billions..."
-the other Doug
Posted by: alan Aug 20 2006, 07:28 PM
The planets were originally naked eye objets which wandered against the stars. If Io, Europa, Ganymede,and Callisto orbited farther from Jupiter they would be visible to the naked eye...
Posted by: JamesFox Aug 20 2006, 07:39 PM
From what I've heard, Vesta can be seen by the naked eye sometimes...
Posted by: climber Aug 20 2006, 09:42 PM
Get late in the debate so may be some body already said it, anyway, since Pluto & Charon are both planets Alan Stern must me very happy : he could have been a PI heading to a, hum, what, asteroïd?, and now he's been upgraded of going to TWO planets (and may be more ) (not couting Jupiter 
)
Posted by: JRehling Aug 20 2006, 11:42 PM
[...]
Posted by: belleraphon1 Aug 21 2006, 02:06 AM
See the following preprint by Steven Soter.....
http://fr.arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf
"Planet as an end product of secondary accretion in a disk around a primary star or substar. Planets in this sense only occur in highly evolved (old) systems..........All planets in our solar system are sufficiently massive to scatter most planetesimals out of their orbital zones"
And they are several orders of magnitusde more massive than any other bodies in the vicinity....
I have to agree with this definition which is basically the same as the alternate definition that has been offered up for vote.
This is a solid quantitative definition that would explain the eight classical planets and religate the rest of the solar system bodies to a small body status.
I have always felt in my gut that there is a fundamanetal difference between Mercury thru Neptune and the rest of the bodies in our solar system.
And so what if Ceres and Pluto are not accored 'planet' status? These are still incredibly exciting 'worldlets' well worth exploring.......... a body does not have to be a planet to be worthy of respect and study....
If we are going to define a scientific term, let us use science, not emotion.
Craig
Posted by: ljk4-1 Aug 21 2006, 02:38 AM
Here is another planet definition:
On the low-mass planethood criterion
Authors: Bojan Pecnik, Christopher Broeg
Comments: 13 pages, 1 table, submitted to journal of Planetary and Space Science
We propose a quantitative concept for the lower planetary boundary, requiring that a planet must keep its atmosphere in vacuum. The solution-set framework of Pecnik and Wuchterl (2005) enabled a clear and quantitative criterion for the discrimination of a planet and a minor body. Using a simple isothermal core-envelope model, we apply the proposed planetary criterion to the large bodies in the Solar System
http://arxiv.org/abs/astro-ph/0608367
Posted by: JRehling Aug 21 2006, 04:22 PM
[...]
Posted by: mcaplinger Aug 21 2006, 04:36 PM
There seems to be some confusion between an objective, quantitative definition, and a "scientifically-motivated" one. The seeming need for the latter is driving people to these tortured definitions involving atmospheres, hydrostatic equilibrium, barycenters, domination of neighborhoods, etc. Maybe I've been consorting with engineers for too long, but what would be so "unscientific" about simply setting a size limit so that Pluto stays a planet and anything larger would be one, and anything smaller wouldn't?
I'm reminded of the effort to define the meter as a fraction of the Earth's circumference before it was actually possible to measure the Earth to the needed level of accuracy. Much effort was spent but in the end the meter doesn't really match the Earth (and they knew this when the definition was made) -- see THE MEASURE OF ALL THINGS by Ken Alder, very interesting read.
I'm also wondering who goes to these IAU meetings. Wouldn't the real scientists stay home and do productive work?
Posted by: David Aug 21 2006, 05:21 PM
I'm glad you asked that question, as I have obtained the following photograph of their secret deliberations that may shed some light on the subject:
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Posted by: Stu Aug 21 2006, 06:10 PM
Glad you explained what it was, I might have thought it was the very first UMSF bar-b-q picture otherwise...
Posted by: AlexBlackwell Aug 21 2006, 06:57 PM
I was going to suggest the same thing (or some nice round number for diameter that retains Pluto) but that seemed too simple to me. After all, who am I to argue with the IAU?
Note also that TPS has a couple of new pieces on this issue. http://www.planetary.org/blog/article/00000677/ and http://planetary.org/radio/show/00000198/.
Posted by: helvick Aug 21 2006, 08:03 PM
OK I've had enough, clearly we need an omipotent dictator to decide on this and conveniently we have a good one here.
Doug - tell us what a planet is and we'll go forth and make it so.
Posted by: djellison Aug 21 2006, 08:16 PM
Ooo - don't let me decide - at least these ideas get some support - I'd come up with something that'd piss EVERYONE off
At least, I'd try to
Doug
Posted by: volcanopele Aug 21 2006, 08:21 PM
A Planet is an object less than 13 times the mass of Jupiter and is large enough to have it shape determined by hydrostatic equilibrium (round or nearly round). no further qualifications.
Posted by: dtolman Aug 21 2006, 08:43 PM
On the low-mass planethood criterion
Authors: Bojan Pecnik, Christopher Broeg
Comments: 13 pages, 1 table, submitted to journal of Planetary and Space Science
We propose a quantitative concept for the lower planetary boundary, requiring that a planet must keep its atmosphere in vacuum. The solution-set framework of Pecnik and Wuchterl (2005) enabled a clear and quantitative criterion for the discrimination of a planet and a minor body. Using a simple isothermal core-envelope model, we apply the proposed planetary criterion to the large bodies in the Solar System
http://arxiv.org/abs/astro-ph/0608367
Why such an esoteric boundary? Why not something nice and simple - average equatorial G of 1 m/s^2 or higher. Or average diameter of 2000 KM or larger?
The difference between minor planet and planet is admittedly arbitrary. Why not use something simple like radius, or gravitational pull to mark the cutoff?
Posted by: David Aug 21 2006, 08:45 PM
And within that group, planets are classified as "Colossal" (Jupiter, Saturn), "Jumbo" (Uranus, Neptune), "Extra large", "Large" (Venus, Earth), "Medium" (Mercury, Mars, Ganymede, Callisto, Titan), "Small" (Moon, Io, Europa, Triton, Pluto, 'Xena'), and 'Peewee' (everything else).
Note that "Extra large" has no members in our Solar system... hmm... I think I see a candidate for "scientific division between planets and non-planets" there. Any extrasolar planets detected yet with masses somewhere between Uranus and Earth?
Posted by: helvick Aug 21 2006, 08:48 PM
OK all jokes aside I'm with you on this one. The barycenter thing may have some convoluted logic to it but "planetness" has to be something inherent in a thing and cannot be dependant on its environment\situation. It is silliness in the extreme to make Charon a planet but not Titan, Luna or Io.
Posted by: JRehling Aug 21 2006, 09:42 PM
[...]
Posted by: volcanopele Aug 21 2006, 09:47 PM
Sorry, hydrostatic equilibrium. Post fixed.
Posted by: hendric Aug 22 2006, 04:45 AM
The IAU executive commitee just needs to grow a pair and state that "Everything above XX km in diameter is a planet, unless it has participated in nuclear fusion." and just tell everyone else to screw off.
Posted by: Holder of the Two Leashes Aug 22 2006, 05:53 AM
So whoever would have thunk it. The IAU duly appoints a small committee to settle the definition of a planet. Said committee duly arrives back with a unanimous verdict, then heads off for fun and frolic. Upon receiving definition-by-committee-with-no-dissent, the general membership of the IAU, by all appearances entirely on its own, starts a major dust up over the matter. Something seems rather unseemly here.
Not only that, but there's been some hand wringing among a few at the IAU over the fact that the moon might transition to a planet, in such a way that you would think it's going to happen this century rather than billions of years down the road. Less than two actual centuries from now, asteroid 3753 Cruithne will go from being an Aten class to an Apollo class. No big deal, it'll just be deleted from one list and added to the other.
A true story here. I myself once asked Professor Tombaugh, in person, about the mass limit of a planet.
It was at the University of Kansas, where I was a student, shortly after the discovery of Charon. He gave a lecture to a packed audience at the student union, during the course of which he stated that Charon had shown Pluto's mass to be "shockingly low".
After the lecture, I got in the long line to meet him. I remember the fellow in front of me (with trembling hands) asked for four special envelops to be autographed, which Tombaugh graciously did. Then I stepped up, nervously cleared my dry throat, and dared to ask where between Ceres and Pluto he would draw the line on planets.
His answer? He didn't have one! He was settled on the idea that, no matter what, Pluto was a planet, but he frankly admitted he didn't know where to draw a line. He mostly pointed out the large gap between the relative sizes of Pluto and Ceres. True enough then, but now we have gap fillers.
So that is that.
One last thing, about short lists for the benefit of kids. When I was growing up, Jupiter had twelve moons (five named), Saturn nine, Uranus five, and Neptune two. Fairly easy to keep track of. Now we have substantially more inner moons, which by themselves would still be concise enough. Then add on the mass of outer moons multiplying like rabbits, and now the situation is impossible for any student who is not obsessive over them (and I'll admit to being a borderline case).
Then end result? Nobody is asking for fewer moons. Kids simply study and remember what seems important. What's important is, first, what has been photographed up close, and second, what stands out.
Phobos and Deimos because they're martian. Io with the volcanoes. Europa cause it might have life. Titan absolutely. Enceladus maybe. Of course, there is also that one with the footprints on it.
My prediction is that the most important planets will be the classical ones, the ones that can be seen.
Mercury, Venus, Mars, Jupiter, and Saturn. Uranus and Neptune will continue to be remembered by every kid serious about astronomy, especially if they have a pair of binoculars. Paradoxically, the fact that Vesta can be seen, barely (and I have seen it naked eye, barely), may insure that it stays an asteroid. That way those blessed with good vision have the hope of seeing, naked eye, an asteroid. This rather than just another faint planet, like Uranus.
It's possible that Ceres may take Pluto's place in peoples affections as a "pet" planet. Who can resist a tiny little giant? After DAWN we will have side by side scale pictures of Vesta and Ceres. I'm expecting that Vesta will look like a very big asteroid, and Ceres will look like a very small planet. And then when New Horizons reaches Pluto, Pluto is going to look an awful lot like a planet.
Pluto will retain it's position in the cultural consciousness both from history, and by the fact that it's currently the only one among the outermost swarm that can be directly viewed with a modest sized telescope.
As for the rest of the planets lurking out there, well... The average student of astronomy will consign them to where they are, relegated to obscurity in the anonymous outer darkness, where there is weeping, wailing, and gnashing of teeth.
Unless we find a really big one. Or a really strange one. Or a really big, strange one.
Lastly, I have no idea what to think of Charon as a planet, although at first I was vaguely against it. Maybe I still am. Not sure.
Well that's my say. Now I'll retire to await the final verdict with the rest of you, and read others opinions here in the short meantime.
Posted by: dilo Aug 22 2006, 07:19 AM
Dunno if someone already highlighted here, but http://www.unmannedspaceflight.com/index.php?s=&showtopic=3077&view=findpost&p=65103 another possible definition of double planet that seems to better match the unicity of Pluto-Charon and (eventually) similar double asteroids/KBO objects.
Posted by: SFJCody Aug 22 2006, 04:27 PM
Debate goes from bad to worse:
http://www.newscientistspace.com/article/dn9817-battle-of-the-planet-definitions-heats-up.html
Posted by: ljk4-1 Aug 22 2006, 05:48 PM
Planetesimals To Brown Dwarfs: What is a Planet?
Authors: Gibor Basri, Michael E. Brown (Univ. of California, Berkeley and California Inst. of Technology)
Journal-ref: Annual Reviews of Earth and Planetary Science, 2006, v. 34, pp. 193-216
http://arxiv.org/abs/astro-ph/0608417
Posted by: Stu Aug 22 2006, 08:36 PM
Looks like the anti-Pluto bullies might have got their way after all...
http://www.newscientistspace.com/article/dn9818-astronomers-lean-towards-eight-planets.html
Posted by: alan Aug 22 2006, 09:18 PM
Authors: Gibor Basri, Michael E. Brown (Univ. of California, Berkeley and California Inst. of Technology)
http://arxiv.org/abs/astro-ph/0608417
Michael Brown: discoverer of the 10th planet sounds much more significant than Michael Brown: discoverer of 20 out of 80 plutons.
Posted by: JRehling Aug 22 2006, 09:20 PM
[...]
Posted by: DonPMitchell Aug 22 2006, 09:36 PM
I still don't see the rationale for demoting Pluto, which has been a planet since the 1930s. Deciding Pluto is too small is just arbitrary. How does one justify keeping Mercury and rejecting Pluto based only on that criterion?
However, it seems this question has come up because of the discovery of KBOs. So now there is a sense that there will be "too many planets". Again, that is arbitrary. What if there are many Mars-sized objects out there? Will we go through through this whole silly exercise again, with dozens of ad hoc definitions of "planet" being proposed?
The IAU doesn't seem to have been any more scientific about this than the hundreds of opinions I've read on multiple forums. It's become a big joke in the media now, so from a public-relations standpoint it was handled very badly by the astronomy community.
Posted by: David Aug 22 2006, 11:19 PM
I'm beginning to realize that there is a real cultural issue here -- but it's not about popular culture. It's about the professional culture of astronomers.
With a few exceptions, it seems that "planet" is a word that many astronomers want to own. After all, if "planet" belongs to some group of people, it ought to be the astronomers, right? So how do you show that you own a word? By putting your own stamp on it -- by giving it the definition that you want. And if it's a definition at variance with common perceptions, all the better -- because that way you show that you are not just following popular thought, but really lending it the benefit of your uniquely scientific thought. But when you invoke that kind of justification, the matter becomes personal. Hence the insistence that one definition is scientific and another is not.
It's been noted that debates become most heated when the least significant and least verifiable issues are on the line. I think that's because those issues allow more of the personal element to be injected into the fray. Perhaps they do need several years to hash this out, although that could also be several years in which opinions become hardened.
And yes, at present the IAU does not seem to be coming out of this smelling like a rose.
Posted by: Stu Aug 22 2006, 11:48 PM
You make some very good points there David, food for thought indeed.
Personally I will be bitterly disappointed if Pluto loses its planetary status. Not only will the IAU be belittling and snubbing the achievements of Clyde Tombaugh, they'll be showing - to some degree - they can be cowed by or at least influenced by aggressive almost bullying tactics. That committee, in good faith and knowing they were on (as we put it here in the UK) a hiding to nothing, put out a proposal for considered debate and discussion, and then certain groups attacked them and their work in a needlessly angry and confrontational way. I think quite a few astronomers have tried to create influence and make names for themselves here with no real respect or passion for the actual issue itself.
If Pluto is demoted I predict two things. 1. There will be an even bigger "Save Pluto" campaign than before, and 2. astronomy, and astronomers, will come out with a very tarnished reputation that will take some time to repair.
Posted by: Stu Aug 23 2006, 12:03 AM
...which, I fear, seems like it would do anything to stop young whippersnappers like Mike Brown, with their new-fangled sky surveying telescopes, get acknowledged as "planet discoverers". Professional science at its worst here, I fear, more about clashes of personalities and character and - perhaps professional jealousies - than doing what's good and best for science and The People.
Posted by: nprev Aug 23 2006, 02:10 AM
Here's a thought that hasn't apparently yet been expressed: What happens to this whole house of cards if we find a Mercury-sized or better KBO? I personally think that the odds are way better than 50-50 that we'll find one (or ten, or a hundred) of such beasts within the next twenty years.
I have to admit that I'm a size chauvinist; the thought of all these small bodies classified as planets seems completely inappropriate. So, how about this: Any spherical body in an independent orbit around the Sun that is equal to or greater in diameter to the largest natural satellite (Ganymede) in the Solar System is by definition a planet?
To clarify: I'm not a Pluto-hater, but let's face it: Pluto's been considered an anomaly since its discovery, and nothing in this debate has really changed that. In this view, the Pluto system still gets to be the crown jewel of the KBO population (at least for now...)
Edit: Rats; forgot that Ganymede is bigger than Mercury, and Mercury is most definitely a planet. Guess that's why I never get invited to IAU meetings...
Posted by: dvandorn Aug 23 2006, 02:23 AM
Seems to me there is another way of approaching the "planet definition" question.
Method of formation.
The inner planets and the gas giants / ice giants were formed by accretion via the dynamics of a rather tightly planar accretion disk. These planets were formed as part of the accretion dynamics of the Sun itself.
the Kuiper Belt and the Oort Cloud are both clouds of matter that seem to surround the Sun in a roughly spherical array. The material out in these clouds do not appear to have been formed within the main accretion disk that made up the original solar nebula.
Yes, I know this would reduce us to only nine planets if you count Ceres, and eight if you don't. But perhaps "planets" are objects formed from the dynamics of the original accretion disk, and everything else is debris that accreted outside of these dynamics? Therefore, a planet would have to occupy the plane of the ecliptic in order to have been born directly from the original accretion disk...?
-the other Doug
Posted by: David Aug 23 2006, 02:29 AM
I have to admit that I'm a size chauvinist; the thought of all these small bodies classified as planets seems completely inappropriate. So, how about this: Any spherical body in an independent orbit around the Sun that is equal to or greater in diameter to the largest natural satellite (Ganymede) in the Solar System is by definition a planet?
Mercury's smaller than Ganymede
-- although due to its unusually high density, it is considerably more massive. If you made it "as massive or more than Ganymede" you'd include Mercury.I say that "planet" is any natural object in a stellar system; that sub-groups among these planets are comets, asteroids, KBOs (although we should find a better name for them), SDOs, terrestrial planets, Jovian planets, and satellites, none of which are defined by size (though they may have typical sizes); that there is some special category, let's say "planete" for objects that are "big enough to be round"; and that multiple membership is possible.
So: Jupiter is a Jovian planet and a planete;
Earth is a Terrestrial planet and a planete
Ceres is an asteroid (minor) planet and a planete
Eros is an asteroid (minor) planet and not a planete
Pluto is a Kuiper belt planet and a planete
1992 QB1 is a Kuiper belt planet and not a planete
Sedna is a Scattered Disk planet and probably a planete
Ganymede is a satellite planet and not a planete
Phoebe is a satellite planet and not a planete
There, now everybody can be a planet! Are we all happy now?
Posted by: Holder of the Two Leashes Aug 23 2006, 03:08 AM
I can't find the exact article, which I believe was on space dot com, but this was addressed once by a Pluto demoter. Since KBOs constitute a "swarm", like the asteroid belt, and there should be a gradation of sizes where nothing stands out by, say, orders of magnitude, then it was the opinion of the writer that none should be planets. Not Pluto. Not a Mercury size object. Not a Mercury mass object. Not the biggest, even if it's bigger than Earth.
If a Mars size object is found out there, there is a segment of astronomers that want it designated as 2009 XB14, for example. Then handed number 301576 when its orbit is established.
I hasten to point out that I don't share this opinion.
Posted by: Greg Hullender Aug 23 2006, 03:46 AM
I'm surprised no one has added criteria like near-zero eccentricity and near zero inclination to the solar equator. When you couple those with roundness and domination, it's apparent that only eight bodies meet all four criteria -- the same eight that met the last two.
Posted by: David Aug 23 2006, 04:02 AM
Except for Mercury...
Posted by: Holder of the Two Leashes Aug 23 2006, 04:04 AM
I believe (someone correct me on this if I'm wrong) that the earth itself has something like a seven degree inclination to the solar equator.
Some of the newly discovered extrasolar Jupiter sized planets have fairly eccentric orbits.
Posted by: David Aug 23 2006, 04:34 AM
Some of the newly discovered extrasolar Jupiter sized planets have fairly eccentric orbits.
Major planet inclinations vary between 3.4° and 7.2° to the solar equator. Not huge, but not insignificant either. Earth's is the greatest.
Quite a few asteroids have both inclinations and eccentricities similar to those of the major planets.
Posted by: volcanopele Aug 23 2006, 05:53 AM
Criteron based on orbital parameters would be one I would be adamently opposed to. As HotTL mentioned, there are quite a few extrasolar planets with highly eccentric orbits, yet are more massive than Jupiter. Inclination is a little more difficult to discern, but there is no reason to suspect that extrasolar planets with highly inclined orbits don't exist.
Posted by: karolp Aug 23 2006, 11:49 AM
Why is demoting Pluto such a big deal? There were lots of such cases - bodies were demoted as our knowledge grew:
http://en.wikipedia.org/wiki/List_of_Solar_System_bodies_formerly_considered_planets
The reason why it is so difficult to agree upon a definition of planet is partly because they used to be simply "wanderers" in the sky to the Greeks and thus something like lunar maria - initially considered to be seas of liquid water and now retained as names for historic reasons. And thus the cultural flavour of the word planet as noted by Mike Brown on his homepage. The other reason is that we know so little about OTHER planetary systems, where there might be jupiters in inclined and eccentric orbits. Thirdly, we know little about the past configuration of our own planetary system - one explanation of Uranus' tilt is that used to pass closely to Saturn in a more elliptical orbit than it is today. All in all it seems more reasonable to demote Pluto and wait for more discoveries in the Kuiper Belt and Oort Cloud as well as in the extrasolar domain to get a feel of what things really are like out there.
And finally: author Dava Sobel is on the committee, probably due to her conections with Carl Sagan and ellegedly due to her expertise in history of astronomy - I say supposedly because if that was the case she would know about Ceres once being demoted and would certainly oppose bringing it up all over again. Just listen to "Author Dava Sobel is making planets" - yeah, why not - just MAKE some (anyone want to make asteroid Itokawa a planet as a nice gesture to the Japanese?):
http://www.planetary.org/audio/avfiles/pr20060821_32kb.mp3
And it is REALLY interesting to know that Dava Sobel is author of http://www.amazon.com/gp/product/0670034460/sr=8-1/qid=1156334019/ref=pd_bbs_1/102-7819918-3018561?ie=UTF8 where "her "Jupiter" essay becomes a meditation on astrology". Congrats to you IAU folks, why not have Nancy Lieder on the committee as well (remember the "Zeta Emissary" of Planet X?). Sure would be fun. Just listen to how Dava Sobel praises astrology (as well as EATING, yeah EATING Martian SNC meteorites) here:
http://www.planetary.org/audio/avfiles/pr20051031_32kb.mp3
What amazes me most is that Mike Brown is not on the committee although he actually DEALS with planetary science and the other folks there (apart from Dava Sobel) are astrophysicists and such and do not care much about the real understanding of planets either.
Posted by: David Aug 23 2006, 12:52 PM
None of those reclassifications, except perhaps that of the Sun, really has anything to do with an increase in knowledge.
Sun – I would hardly call this a "demotion", but its reclassification is part of the shift from a Ptolemaic (or Tychonic) to a Copernican-Keplerian system, which -- you may recall -- was a very big deal.
Moon, Io, Europa, Ganymede, Callisto – Throughout the 17th century, the term "planet" also included moons (Titan, Iapetus, and Rhea could be added to the list), as indeed there was originally no other term for bodies with a visible motion independent of the "sphere of the fixed stars". Later on the term was superseded by "satellite". This was a linguistic, not a scientific development, although it is generally (not always, as we see with the Charon instance) pretty easy to tell if something is a satellite or not.
Ceres. Pallas, Juno, Vesta – Of course, this reclassification is part and parcel of the whole question of the distinction between "planets" and "minor planets" which is being revisited today. The reclassification was part of a bookkeeping exercise, and did not reflect an increase in knowledge so much as a frustration at the accumulation of asteroids and what it was doing to the nice neat planetary tables of almanacs!
That is a hasty and illogical conclusion. It is quite possible to know all about Ceres' "demotion" and -- indeed, in light of increasing knowledge of Ceres' size and shape -- to think that it was the wrong decision. In any case, if one adopts a "roundness" definition of planet, one cannot be consistent and reject Ceres' planetary status. The "roundness" definition was not proposed by Sobel.
I find the hostility towards Sobel, and the apparent implication that the work of the IAU committee is somehow tainted by her presence, inexplicable. The following seems quite unwarranted:
What I heard was a popular historian who is aware of the significant cultural role astrology played, and its close links with astronomy down to the 17th century. Astrology has no claim to be a science, but no cultural historian can ignore it when dealing with the astronomy of the 1600s and earlier. In any case, whatever Sobel's views on astrology, there is absolutely no reason to suppose that the IAU committee proposal had anything to do with them (I get the impression that Sobel contributed little to the committee decision, and was there mostly for the PR). If it were up to the astrologers, I'm sure we'd find ourselves back with the seven Ptolemaic planets and Earth in the center of the universe!
Posted by: karolp Aug 23 2006, 01:30 PM
Demoting Sun - awareness of distinction between Sun (and stars) and planets - even if initially based only on the relative motion or rather the lack thereof (followed by awareness of gravity and nuclear fusion). Demoting Europa & Co. - awareness of such thing as planetary satellites. Demoting Ceres & Co. - discovery of the asteroid belt as a belt - harboured by many such bodies. And... demoting Pluto following the discovery of the Kuiper Belt as a belt comprising lots of such small icy bodies, with the awareness starting to build up in early 1990s after the discovery of 1992QB1.
Demoting Ceres was the result of realisation that there is an asteroid belt between the orbits of Mars and Jupiter comprising lots of such small bodies. Promoting Ceres back to planetary status denies that crucial part of understanding of our planetary system.
What I heard was a popular historian who is aware of the significant cultural role astrology played, and its close links with astronomy down to the 17th century.
I love history of astronomy myself and I love the historians of astronomy (and of anything else) as well. And I am aware of the initial close connection of astronomy and astrology prior to the 1700s. But I hope you would agree that taking the astronomy-astrology connection over into the 21st century is just ridiculous and wrong in so many ways. And I do find myself deeply worried and disturbed when someone actively praising astrology and its "merits" sits on an IAU committee whose decisions will affect the astronomy textbooks AND the PR of planetary science in the years to come.
Posted by: Stu Aug 23 2006, 01:33 PM
Sorry, I'm not having that.
Dava Sobel was on the Committee, I think, precisely because she's NOT an astronomer, and also, I think, because she "gets" the solar system and its place in our history and culture. Her book "THE PLANETS" is an outstanding work, a refreshing change from the countless fact-engorged samey samey reference works weighing down the shelves of the Popular Science sections in the world's bookstores. It's effectively one of the longest and most beautiful astronomical poems I've ever read. She clearly isn't a believer in astrology, she was just explaining its place in the development of astronomy as a science and the evolution of our understanding of, and relationship with, the Solar System. Using astrology as the hook to hang her "Jupiter" chapter on was just being different. The "Mars" section is written by a talking meteorite from Mars - are you suggesting she actually believes meteorites from Mars can talk?
Criticising her presence on the committee is, in my not-worth-a-damn-I-know opinion, ridiculous. I have no doubt whatsoever that when all those professional astronomers were in danger of vanishing, oozlam-bird style, up their own backsides with their dry definitions, astrophysical gobbledygook and technobabble, she was able to butt in and put across a more human and emotional point of view. In effect she was our - that's the astronomy enthusiasts' - representative there, detached from the intellectually cannibalistic world of professional astronomy. The Committee can only have benefitted from her being on it.
Posted by: karolp Aug 23 2006, 01:47 PM
I am not having that either. Since when classification of planetary bodies is based on an "emotional point of view" as opposed to "technobabble"??? Are we going to leave Pluto as a planet and promote Ceres back to planethood becasue we LIKE them??? Are we going to ignore the existence of asteroid belt and Kuiper belt becasue that is "technobabble"? And what exactly do you mean by "intellectually cannibalistic"
Back on more reality-based level, I do second your opinion that "planethood" lamentably still IS based more on cultural views rather than actual science and that we do not yet fully understand what a planet is because we do not put much money into research on other planetary systems apart from our own. And in science a view based on only one specimen is always doubtful.
Posted by: David Aug 23 2006, 01:57 PM
People were aware of the existence of satellites since 1610, and arguably since 1543 (when Copernicus postulated that the Moon was the one body in the Solar system not moving in a heliocentric orbit). The change in name had nothing to do with an increase in knowledge about their nature.
When the reclassification of asteroids began in 1851, only about 20 asteroids had been discovered; enough to make bookkeeping difficult, but too few to make conclusions about the existence of a "belt". And in the initial stages of the process (which took decades), Ceres was not "demoted"; for a while it enjoyed an intermediate existence, listed both among the "major planets" and also the asteroids. The key factor in its definite reclassification as an asteroid was the assignment of a number to it, replacing the formerly planetary symbol. This was, again, a bookkeeping exercise, not a scientific one.
I don't see why that should be in the least bit true.
I'll let Stu's comment stand for what I might have said in response to that.
Posted by: Stu Aug 23 2006, 02:03 PM
I'm firmly grounded in reality thanks, just putting across my point of view after yours. That you chose to mock my post was disappointing but sadly not out of keeping with the tone of some of the input from astronomers involved in this process.
I wasn't suggesting planetary status should be based on emotions, but that they should be considered. Sometimes dry science has to be hydrated a bit with a sprinkling of common sense and humanity. Demoting Pluto won't actually achieve anything, except offending the memory of Clyde Tombaugh and confusing people even more. It simply wouldn't hurt to just leave Pluto the hell alone, with everyone knowing it's not, in the strictest scientific terms, a planet. We'd all know it's an honourary planet, but so what? make an exception for it! Yes, just do it! It's been thought of as a planet by everyone out there for seventy odd years, why mess with that? It would gain astronomers, and the astronomical community, so much goodwill to just be honest and say "Ok, well it's not really a planet if we're going to be pedantic about it, but we figure why cause trouble for the sake of it?" People would appreciate and support a bit of humility and good old common sense here.
And by "intellectually canibalistic" I meant that this debate has shown, horrendously, that the world of professional astronomy is just as rife with people trying to score points against each other, ruin careers and reputations and puff up their own chests as other sciences. I used to think it was different, nobler somehow, that because astronomers studied, and "got" the Big Picture it might give them some humility. Guess not. When all this is finished - this time, for I fear there will be a fudge tomorrow - the man and woman in the street is going to be left with the view that astronomers are nit-picky boffins who just like messing with things for the sake of it, and argue amongst themselves. And why? Because we've done a frankly crap job of communicating to people just why this debate is so important, why it has repercussions for the future of science, and why it should matter to Mr or Mrs Average. It's come over as self-indulgent and very badly organised.
When the dust settles, whatever the outcome, astronomy will be the loser, and that makes me sad.
Posted by: karolp Aug 23 2006, 02:12 PM
Sure, why don't we promote a bunch of asteroids we LIKE to planethood due to emotional reasons. I am all in for Itokawa. I really like that ol' stony rubble pile.
There is nothing wrong in bringing human aspects and science together. They often mix well in science fiction art of all kinds which I really enjoy. But mixing them too much simply creates a lot of nosense. And denying the KBO status of Pluto as well as re-promoting Ceres is a nice example of such nonsense. The same applies to Charon - actually the baricenter of the Jupiter-Sun system also lies outside of the Sun's surface. But that does not make the Sun a double planet, does it? Unless you REALLY like Jupiter and would like to give it a nice promotion. Let's just wait for tomorrow's outcome. And if they make 9 planets 12 to even the number nicely I might as well eat a talking ;-) SNC meteorite. But I will also demand that the IAU be named International Astrological Union ;-)
Posted by: Greg Hullender Aug 23 2006, 02:13 PM
volcanopele: May I ask why? These all seem to me to be things related to the original formation of the system.
Let me relate this back to my own field, pattern matching/machine learning, because I see it as a classification problem. (It's hard to argue that this isn't about classification.) :-)
If you look at the distribution of bodies in solar orbit by mass, inclination, and eccentricity, I suspect you see something that looks like two smooth distributions superimposed on each other, with a few spikes. The two distributions would be the Asteroid Belt and the Kuiper Belt, and the spikes would be the eight planets. Without using distance from the sun as a parameter, there would be a lot of overlap between the two belts (I believe KBOs have far more variation in inclination). If we include distance, though, the separation should be even more stark.
It would be fun to play with the data (is there a handy single source that has this info in it?) and see for sure, but I'll bet all eight planets are more than three sigmas away from the mean of either major distribution, and I'll bet no other body of nontrivial mass (except the sun) is. (More formally, I believe the simple statistical model will predict that no planet has any significant probability of being an asteroid or a KBO [except Pluto], but every other body of significant mass falls into one or ther other category.
Distributions like this tend to arise from natural processes; they're not arbitrary. Something happened during the formation and evolution of the solar system to produce these distributions and these eight exceptions to them.
The near-perfect power-law spacing of the ten entities suggests this as well. I get an r^2 fit of 0.9933 on the logs of the semi-major-axes of the 8 planets [using Ceres for the Asteroid Belt] and I find myself wondering if the center of the Kuiper Belt is going to end up at about 49.7 AU.
To me, the data strongly suggest that the Solar system consists of one sun, eight planets, two asteroid belts, and a small handful of leftovers. But to make the distinction between the distribtions sharp, I do think you need to include some of the orbital parameters.
As for extrasolar systems with different distributions, I suspect the super-Jupiter with an eccentric orbit will still stand alone.
Posted by: karolp Aug 23 2006, 02:27 PM
I prefer to view Clyde Tombaugh's achievements as doing 1990s science in 1930s - discovering a KBO well ahead of the mass discoveries that followed years after that. And I really doubt that he would put any personal or emotional reasons above those of scientific merit. From what I read here he initially firmly regarded Pluto a planet but that was years before the discovery of another KBO which happened in 1992.
And it is exactly that humility and common sense that made astronomers demote Ceres in 1850s when miriads of other minor planets started to emerge in its vincinity ;-) And its discoverer, Piazzi died in 1826 but apparently then there were no claims that it might disgrace him in any way. And I do not really think he would feel disgraced by adjusting the classification of Solar System to the increasing understanding of the Solar System and the Universe we live in. And I do not think Tombaugh would mind either. I guess they were both all for science, not astrology.
Posted by: David Aug 23 2006, 02:30 PM
The IAU didn't suggest plucking Ceres from asteroidal obscurity because they liked it, but because it was gravitationally pulled into a rounded shape.
My point, however, was that there's absolutely no reason to think that calling Ceres a planet or calling Pluto a planet will make people forget that they are members of the Main Belt or the Kuiper Belt, or forget that the Main Belt or the Kuiper Belt exist. Quite the reverse: Pluto, and 2003 UB313, draw attention to the existence of the Kuiper Belt which otherwise might be considered a forgettable pile of rubble at the edge of the Solar system.
Posted by: karolp Aug 23 2006, 02:49 PM
And so were lots of other small bodies. Roundness alone is not a good criterion. It is like calling every 4-legged animal a horse. And even if it is a fly who lost 2 legs, it still would be a horse, right? ;-) Basing the classificiatin on roundness does help to preserve Pluto as a planet but also adds dozens more "planets". And also, they would need to determine the roundness of EACH AND EVERY KBO no matter how distant to actually say how many planets there are in the Solar System
But it would be hopellessly artificial. I guess we might distinguish large round bodies like Ceres, Pluto or Xena by calling them planet-oids but calling them planets is like promoting randomly picked flies to be horses :-) And calling KBOs planets actually draws the public's attention AWAY from the Kuiper Belt as they do not need to bother with the rest of small bodies there as they are happy remembering the existence of a handful of their nice round planets.
Posted by: JRehling Aug 23 2006, 03:46 PM
[...]
Posted by: JRehling Aug 23 2006, 04:02 PM
[...]
Posted by: David Aug 23 2006, 04:21 PM
But it would be hopellessly artificial. I guess we might distinguish large round bodies like Ceres, Pluto or Xena by calling them planet-oids but calling them planets is like promoting randomly picked flies to be horses :-) And calling KBOs planets actually draws the public's attention AWAY from the Kuiper Belt as they do not need to bother with the rest of small bodies there as they are happy remembering the existence of a handful of their nice round planets.
Why isn't roundness as good (and as bad) a criterion as any other? What's intrinsically wrong with more planets? What's not right about a closer telescopic (or UMSF) investigation of as many KBOs as possible?
It seems to me that saying "calling KBOs planets actually draws the public's attention AWAY from the Kuiper Belt" is like saying that calling certain administrators "cabinet members" draws the public's attention away from the Executive Branch. I do not think anyone really thinks like that.
1) Saturn-Uranus, ratio of 0.424
2) Neptune-Earth, ratio of 0.258
3) Mercury-Xena, ratio of 0.492
4) Pallas-Juno, ratio of 0.471
[others on the tiny end?]
Also note the smaller, but still quite large gaps: Mars-Venus: .562; Mercury-Mars: .717 . There's also Vesta-Ceres at .593, but there's a whole lot of KBOs that fit in that gap. If you count the satellite bodies Callisto, Io, the Moon, Europa, and Triton in between Mercury and 2003 UB313, the "Mercury-Xena" gap quite disappears; the biggest gap left is Callisto-Mercury at a not-too-significant .759. Pallas-Juno isn't even a legitimate gap; the fourth largest asteroid is Hygiea. Juno's about the 12th largest. In the smaller sizes I doubt there's a gap under .9.
Posted by: volcanopele Aug 23 2006, 04:23 PM
The problem, Greg, is that while objects forming around stars may start out with relatively low inclinations and eccentricities, they don't need to end up that way. We have observed extrasolar planets with high eccentricities. See http://jilawww.colorado.edu/~pja/planets/extrasolar.html for a graph showing eccentricities of observed extrasolar planets. As you can see, most of the ones we've seen have higher eccentricities than the "classical" planets. Are all these objects no longer planets because of this fact. (I leave out inclination from this discussion simply because it is harder to observe inclination from the available data). So while such a definition could arguably work in our solar system, it would not work for the majority of extrasolar planets observed thus far.
Posted by: Michael Capobianco Aug 23 2006, 05:25 PM
Can we reliably say that, no matter how the IAU chooses to define "planet," we'll have official names for "Xena" and "Gabrielle" by the end of the week?
And is the approval of names for Cassini-discovered features of the Saturnian satellites on the agenda?
Michael
Posted by: Michael Capobianco Aug 23 2006, 05:26 PM
I still can't think of better terms than planet, minor planet, and a new designation, "major minor planet."
Michael
Posted by: JRehling Aug 23 2006, 05:42 PM
[...]
Posted by: volcanopele Aug 23 2006, 06:17 PM
And is the approval of names for Cassini-discovered features of the Saturnian satellites on the agenda?
Michael
I wouldn't get your hopes up on either case. If "Xena" is given planet status, then it may take another committee to pick a name for it. If it isn't, then a new name may come faster since Mike Brown has already submitted a minor planet name for it.
As for names for features on Saturn's satellites, the Titan and Phoebe names should be formally approved. For the other satellites, I have no idea where the names are in the process. They weren't given provisional approval at the very least, and I have no idea if they need provisional approval by the WGPSN. I know they are usually provisionally approved by that working group, and then are formally approved by the IAU Division III during the General Assemblies. If the provisionally approved step is necessary (prior to the general assembly), don't expect new names for features on the other icy sats.
Posted by: karolp Aug 23 2006, 06:54 PM
Good point. Actually the continents are in a sense defined by tectonic plates - Europe used to be a separate plate (the Ural mountains are where it was "stitched" to Asia) but so was India and we do not call it a continent (although the term sub-continent is sometimes used). But sub-continent makes about as much sense as "dwarf planet". And I think it is also a good point about throwing the ball away no matter what. I do not consider inviting non-astronomers to decide to be a "good" way of public involvement. It only shows that astronomers are helpless at it themselves. And the public is reported rather to laugh at the 12-planet proposal and rather than being a good PR it rather undermines astronomers' credibility:
http://www.space.com/scienceastronomy/060818_planets_rxns.html
"Planet" has not grown far beyond its cultural meaning because we do not understand planets well - we only know ONE planetary system in detail - being our own. And hence eccentricity or inclination are not good criteria because of jupiters and neptunes in elliptical, inclined orbits - and presumably our own Uranus as well (in the past). We are too Sol-centered to decide upon the definition yet. We are not ready. But we do realise what the Kuiper Belt is. And thus we may at least clarify things with Pluto. Leaving it "as is" as proposed by Stu does not really disturb me that much as re-promoting Ceres and messing with Charon. If in a given zone there is no large dominant body formed, you are left with an asteroid belt. And Ceres came closest to becoming that dominant body but failed - otherwise the belt would have been cleared. THIS is what is wrong about calling Ceres a planet.
And there is nothing intrinsically wrong with having many planets as long as they are there. Promoting arbitrarily chosen asteroids and KBOs to "make" them planets IS wrong and clearly desperate. It is like inflation in economy - if you are paid in thousands, money becomes worthless. If anything can be a planet if we say so, then nothing is.
Posted by: dilo Aug 23 2006, 06:59 PM
I think too, roundiness alone isn't a good criterion, consider as extreme example Saturn which shape is strongly deformed from centrifugal force (yes, we could transform "spherical" word in "equipotential surface" but this is a less intuitive concept).
However, I think you misunderstood: the real criterion in the committe remain the size (or, even better, mass)! We do not need to measure the shape of every object in solar system, this would be really crazy... they introduce the roundiness criteria only to justify an otherwise arbitrary size limit! This sounds reasonable.
Unfortunately, this "objective" size threshold do not fulfill the initial (untold) target to promote Pluto and take away smaller objects, historically considered planetesimal or asteroids or KBO...
And I suspect THIS is the main source of conflict. At this point, I think isn't worth to discuss again and again because there are historical, cultural and (perhaps) political issues related to the change. We already have enough real, stupid wars around (oops!) and the last considerations from David convinced me that best thing is to leave untouched historical planet designation and put all remaining objects in other cathegories (unless we discover a Earth sized KBO!).
Perhaps I'm slightly tired by discussion, but at this point I vote for the "status quo"!
Posted by: karolp Aug 23 2006, 07:11 PM
Yeap, determining absolute roundness of each individual KBO would not only be tiresome and unproductive but actually next to impossible with the current resolution of even the largest scopes. And from astronomical point of view it is not that important. But MASS would not be good either - it can only be precisely determined if an object has a moon orbiting it.
Leaving the status quo would be far better than making the laughable "12-planets thing", but it would still leave that unpleasant feeling of ambiguity and "cheating" as Mike Brown called it
But leaving CERES as is, as well as Charon and Xena and demoting Pluto would make way for embarking on a search for a REAL 9TH PLANET - there might still be a jupiter or at least a neptune hiding in the outer reaches of Oort cloud (ejected from inner Sol system or even captured from another one!). Cleaning up the mess and embarking on a new search for the real stuff - I believe that would bring much more sanity to the issue.
Posted by: volcanopele Aug 23 2006, 08:05 PM
Assuming status quo+"Xena", we could also begin searching for the 11th, 12th, etc. planet.
Posted by: David Aug 23 2006, 08:46 PM
Space.com - Public Laughs and Shrugs at 12-Planet Proposal
What disturbs me is not that people are laughing at this astronomical predicament - I myself find it a source of no small amusement - but that people are taking it seriously to the point of being angry. It is not a scientific issue, or one with any essential significance. It is, at best, an issue of bookkeeping convenience, but more probably one purely of aesthetics. There are a great many possible applications of the word "planet", some of which fit more or less well with historical usages, but none of them is right, and nobody is stupid or wicked for preferring one to the others. The comedians have more of a sense of proportion with regard to this question than some of the astronomers. I have enjoyed the debate so far, but some of the responses are beginning to acquire a tone of self-righteousness which I think is wholly inappropriate. I can live with 8, 10, 12, 20, 50, or an indefinite number of planets. But I would prefer not to be bullied into accepting one version or another on the spurious grounds that one and only one definition of "planet" is the True Scientific one.
Posted by: Stu Aug 23 2006, 10:28 PM
Personally I'm taking this seriously because 1) I thought the attack on Dava Sobel was unfair and unjustified and based on a misunderstanding of her views, 2) this is threatening to make astronomers, and astronomy itself, a laughing stock, and 3) it is going to - in fact, it has already started to - impact on the Outreach work I do, and, I'm sure, many others do too.
Basically, based on conversations I've had with people before, during and now almost-after this "debate", I just know that I'm going to get hassle and, well, not ridicule, but definitely good natured sniggering about this. This past week wnenever I've been talking about something, maybe a new MER picture, a new Cassini pic, the first direct observation of dark matter etc, what have I got back? "Ha, how can I believe all that when you don't even know how many planets there are?"
So, I also just know that when I stand up at the front of a room to talk to a group about something - the search for ET signals, the hunt for life on Mars, the Return to The Moon etc - some *****ole is going to heckle me about "not knowing how many planets there are". Whenever I do my solar system talk, I'm going to get stick for astronomers "changing their minds" about Pluto... and it goes on and on...
I know science moves forwards, things change, ideas evolve and develop and mature. The old is tossed out, the new embraced with open arms... until it in turn becomes The Old and then out it goes into the snow... but Pluto doesn't need to be demoted just for the sake of it, which is what is basically happening I feel. What's wrong with just leaving it alone because everyone already thinks of it as a planet? Seriously, can't we just grant it honourary planet status and move on from that? Everyone Out There "knows" Pluto is a planet. Why? Because we've been TELLING them that! We have! Now we're going to tell them we've changed our minds, it's not a planet after all, never was... well, excuse me while I bang my head against the wall because that's exactly what I feel like doing. It's like someone suggesting renaming the pyramids, or suggesting whales really should have been called "fish" all along.
We're meddling here to make ourselves look clever and brainier than the man or woman in the street, and that smacks of arrogance to me. If this is nothing more than a "book keeping exercise" then shame on us, because that's not a good enough reason for unravelling seven decades of history and public perception.
Posted by: volcanopele Aug 23 2006, 11:17 PM
The final text for tomorrow's votes is out:
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation ‘planets’. The word ‘planet’ originally described ‘wanderers’ that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that planets and other bodies in our Solar System be defi ned into three distinct categories in the following way:
(1) A planet[1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and ( c ) has cleared the neighbourhood around its orbit.
(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape[2], ( c ) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects[3] orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
Insert the word “classical” before the word “planet” in Resolution 5A, Section (1), and footnote 1. Thus reading:
(1) A classical planet[1] is a celestial body . . .
and
[1] The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
The IAU further resolves:
Pluto is a dwarf planet by the above defi nition and is recognized as the prototype of a new category of trans-Neptunian objects.
The following sentence is added to Resolution 6A:
This category is to be called “plutonian objects.”
Posted by: punkboi Aug 24 2006, 01:00 AM
Just read those resolutions...
Weak. I was rooting for the planet Ceres.
Posted by: volcanopele Aug 24 2006, 01:17 AM
According to the source of this text, http://astro.cas.cz/nuncius/nsiii_09.pdf , the most important vote will be for Resolution 5b. If one were to vote for Resolution 5a, but not 5b, you would support the Fernandez et al. position that Pluto is just a dwarf planet, and a not a full-fledge planet. If one were to vote for Resolutions 5a and 5b, you support the 12 planets and counting model, with 8 "classical" planets as well as several "dwarf" planets. In that case, I would vote for resolution 5a and 5b (if I could).
Posted by: vexgizmo Aug 24 2006, 02:37 AM
volcanopele seems to be referring to the "for" and "against" text that accompanies the resolutions. Here it is from that pdf. The accompanying cartoon is the best part.
250 words for
Compromise. Achieving a planet definition has been all
about compromise. There are two equally valid descriptions
of what should be the principal criterion for defining a
planet. One is dynamical, an object that has “cleared out
its zone.” The other is based on the physical nature of the
body itself. The pendulum of argument has swung both
ways during the General Assembly discussions. But now
it has swung too far.
Resolution 5B is all about finding the middle ground.
Using qualifiers gives equal status to both points of
view and leaves open the possibility to define other
types of planets in our Universe. Resolution 5B restores
the “global and cultural points of view” that the Planet
Definition Committee had responsibility to achieve. The
public recognizes Mars, for example, as a “planet” not
because it has cleared out its zone, but because it is a
fascinating world.
To illustrate why Resolution 5B is cultural, and not silly
semantics, consider how you must answer two questions:
How many planets are there? Is Pluto a planet? A vote
in favor of 5B yields: “There are 8 classical planets and
many dwarf planets yet to be discovered” and “Pluto is a
planet, but in the dwarf planet category.” These answers
highlight and communicate the tremendous revolution
of new discoveries in our outer Solar System. Further, it
saves enormous public backlash by still being able to say
the words “Pluto is a planet, but”. Do not underestimate
the global cultural importance of these first four words.
The word “planet” deserves to be shared equally.
250 words against
Resolution 5B represents a small but significant change to
Resolution 5A.
The key issue is the definition of “planet”. Resolution 5A is
close to the version agreed by consensus on Tuesday evening
where it was made clear that three distinct categories of
objects orbiting the Sun were being defined: planets, dwarf planets,
and small bodies. The logical implications from the
rules of grammar cannot be ignored. By using the name
“planet” with two different adjectives “classical” and “dwarf”
a larger category of planets is implied. This contradicts
the first paragraph of both Resolutions 5A and 5B and
transforms three distinct categories into two (planets and
small bodies) and two sub-groups of planets.
To the question “is Pluto a planet?” the two resolutions
give different solutions – “Yes” for 5B and “No” for 5A. To
the question “How many planets are there?” Resolution 5A
gives 8, Resolution 5B currently gives 12 and soon at least
50.
The total number of planets may not matter to scientists,
it is critical for education and the dissemination of science.
For scientists, it is relevant that dynamical and cosmogonical
criteria, which are now the source for the definition of
planets, would in Resolution 5B be relegated to a secondary
role. In Resolution 5A the arguments from geophysics and
from dynamical astronomy are given equal weight. Such a
balanced solution had received very strong support in the
meeting of Division III (Planetary Systems Science) and the
Planet Definition Information Meeting.
Resolution 5B is misleading and should be rejected.
|
Posted by: Holder of the Two Leashes Aug 24 2006, 03:37 AM
I guess we must no longer consider Jupiter a full fledged planet. One look at a map showing the distribution of the Jupiter trojan asteroids, hundreds and hundreds of them, will clearly show that Jupiter has not "cleared out its neighbourhood around its orbit".
Posted by: Stephen Aug 24 2006, 03:40 AM
1) Does anybody know what the IAU's Resolution 5A is referring to by "has cleared the neighbourhood around its orbit"?
Presumably they are referring to the situation seen in the asteroid belt. On the other hand a body surrounded by ring material and/or numerous satellites might also be said to have not fully "cleared the neighbourhood" around its orbit.
Then there is the issue of just how fully a body is supposed to have "cleared the neighbourhood around its orbit" before it is elevated to the status of "planet". To take an obvious example, Earth's orbit is approached or even criss-crossed by assorted asteroids. Some of them might be said to be more or less resident for the time being in Earth's general neighbourhood. Others are frequent visitors to that neighbourhood. Do the presence of such asteroids mean that the Earth has not yet "cleared the neighbourhood around its orbit"?
2) I note that the definition for "dwarf planet" does not define "satellite". Since technically speaking bodies like "Pluto"and Ceres are satellites of the Sun, that could be construed to mean that strictly speaking the Sun has no "dwarf planets"! Merely "Planets" and "Small Social System Bodies".
3) What is the betting that in normal everyday conversation the word "dwarf" is going to vanish from in front of the word "planet", and we are all going to go back to speaking (or start speaking as the case requires) of the "planet Pluto" or the "planet Ceres" rather than the "dwarf planet Pluto" and the "dwarf planet Ceres"? (The alternative would seem to be to start referring to them as the "dwarf Pluto", but that kind of makes them sound like escapees from Tolkien. )
4) Equally not even astronomers and planetologists are not going to be going around calling asteroids and KBOs "Small Solar System Bodies" except perhaps in their scientific papers. If there is to be an SSSB category they need to be come up a term that is less of a mouthful.
5) Overall, this latest debate by the experts on how-to-define-a-planet with its various attempts--and in particular http://www.space.com/adastra/adastra_planet_def_060822.html "under the new definition, there could soon be dozens of new '‘planets' in our solar system. That struck many astronomers as the wrong result"--merely illustrate http://www.unmannedspaceflight.com/index.php?showtopic=2160&st=45# on a different thread of UMSF.com back in April: that behind this whole "what is a planet?" debate lies an element of terrestrial snobbery.
I cannot help feeling the only reason this debate has arisen at all with "planet" is because Earth just so happens to bear the label "planet"; and despite all that has come and gone there is still a subconscious wish, even amongst some astronomers, for Earth to be part of a group with a certain degree of...exclusivity.
Nobody seems to mind seeing the word "moon" used to describe objects ranging from giants like Ganymede and Titan to irregular midgets like Epimetheus and Amalthea yet the notion of having the word "planet" encompass objects the size Pluto as well as those the size of Earth and Jupiter seems to generate a strange unease in some parts of the astronomical and planetological communities.
As I went on to add in that previous post:
[I]t is all right for Jupiter to be called a "planet". That then puts Earth among the giants.
Which begged the question:
[I]f Earth had been a moon rather than a planet would we now be arguing over whether to admit Janus to the hallowed ranks whilst not giving two hoots about using "planet" for Pluto and Ceres?
These latest definitions merely illustrate that Occam's Razor is being cast aside in favour of contrivances of varying degrees of complicatedness whose sole purpose is to keep the word "planet" in some way "exclusive". They have no problem with there being dozens or even hundreds of moons yet they balk at there being dozens of planets in the solar system.
In that context the issue of Pluto is in some respects a furphy that has muddied the waters. Those who want Pluto to be numbered among the chosen few try to contrive a definition sufficiently wide to accommodate it. Those who don't try to contrive one narrow enough it winnow it out, without at the same time risking demotion for more favoured astronomical objects. This in turn leads to varying unease, either at demoting a loved one from the family circle or at the cost of allowing an interloper to stay (some of the riff-raff which got booted into the outer darkness a couple of hundred years would have to be admitted back into the family circle again).
Yet the bottomline seems to be that not even those who want Pluto as part of "Club Planet" want to see dozens of other objects joining it.
======
Stephen
(EDIT: some minor editing to fix a few too many grammatical blunders.
)
Posted by: punkboi Aug 24 2006, 04:10 AM
Don't forget Neptune and its asteroid buddies.
Posted by: mchan Aug 24 2006, 04:35 AM
Agreed that "cleared the neighborhood around its orbit" could have been more strictly defined.
For example, neighborhood could mean the region bounded on the inside by a circular (e=0) orbit with period equal to 1/2 of the period of the candidate object, and on the outside by a circular orbit with period equal to 2 times the period of the candidate object. This example does not work for Venus and Earth, but perhaps a case could be made for some other ratios of orbital resonance such as 2:3 inside and 3:2 outside.
As for the clearing part, I recall Michael Brown's earlier webpage on 2003UB313 had listed the "population in neighborhood" proposal with a discussion of the candidate object's mass. I don't recall if Brown had a specific definition of mass ratios, so an ad hoc example here could be that the candidate object mass must exceed the combined mass of all other objects within its neighborhood. Note that objects within the neighborhood should probably include objects with orbits that do not lie entirely in the neighborhood with some limits, e.g., the object's orbital period must be within some ratio of the candidate object's orbital period, with either the object's perihelion or it's aphelion being within the neighborhood.
The rationale for a neighborhood bounds would be that over cosmological timescales, objects within the bounds would either have coalesced or collided to form the candidate object.
Posted by: mchan Aug 24 2006, 04:45 AM
I like the name of the link on Michael Brown's home page -- "Astronomers are revolting!"
http://www.gps.caltech.edu/~mbrown/
Posted by: David Aug 24 2006, 07:10 AM
Taking a second look at the figures, I note that Pluto (and, tacitly, 2003 UB313) really do deserve some of the ink spilt on them. They really are liminal objects. It has been pointed out, ad nauseam, that they are extraordinarily small for "planets". What is much more rarely pointed out is that they are extreme outliers as KBOs, being many times larger than the vast majority of such objects. They may be "dwarf planets", but they are gigantic KBOs.
Has anyone ever done a back-of-the-envelope calculation to estimate what percent of mass of the entire Kuiper Belt is tied up in Pluto and 2003 UB313?
Posted by: Stephen Aug 24 2006, 07:45 AM
One might as well claim that objects like Metis and Amalthea are "extraordinarily small" for moons.
If the IAU is going to make Club Planet an exclusive preserve for a select few then by rights it ought to do the same for Club Moon as well. After all how many of the solar system's moons would fit the IAU's planet requirement that they have "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape"?
Maybe then we'll start hearing about "moons", "dwarf moons", and "Small Solar System Bodies (Moons)".
======
Stephen
Posted by: David Aug 24 2006, 07:59 AM
Exactly nineteen. Eighteen if you dismiss Charon for one reason or another. S (2003 UB313)1 is a possible but marginal candidate.
Not an entirely absurd idea, though, as you imply, not one that astronomers have felt a great deal of need for.
Posted by: Stephen Aug 24 2006, 09:34 AM
Not an entirely absurd idea, though, as you imply, not one that astronomers have felt a great deal of need for.
Except that a better alternative to "dwarf moon" already exists: "moonlet".
The same might also be said for "dwarf planet": "planetoid" would be a better choice. (Or alternately "asteroid".)
======
Stephen
Posted by: Stephen Aug 24 2006, 09:38 AM
Oh, dear! That sounds like way too many members for Club Moon to have.
======
Stephen
Posted by: AndyG Aug 24 2006, 10:24 AM
...then the Main Belt will be fearing the IAU's wrath.
Andy
Posted by: ngunn Aug 24 2006, 10:46 AM
No mention of the dreaded barycentre now but presumably it's still lurking there in 5B. What will we call a really big trans-Neptunian when we find one? Not classical for sure, since that's presumably a historical term. Giant dwarf planet perhaps? A definition valid for the solar system only is surely a very short-sighted idea at a time when at last we are acquiring the means to explore other systems. For all these reasons and others I'm with JRehling. They really should cut their losses and simply agree that there is no consensus. Science would lose nothing by this. As to Stu's problem with the 'man in the street' I see no difficulty at all. Everyone is welcome to use the word planet as they wish, the man in the street included. As a man in the street myself I would much prefer this to being told what to think and how to speak by a committee. All of their proposals are scientifically pointless and culturally patronising, in a word pompous. Unfortunately they will probably go ahead anyway.
When exactly do they vote?
Posted by: Rob Pinnegar Aug 24 2006, 01:15 PM
Well, you could always call the big ones "moons", and the small ones "satellites". Only thing is, this would make the recently coined term "moonlet" obsolete. "Satellitelet" don't have the same ring.
I tend to think of "moons" in terms of the "Big Seven": the Galileans, Luna, Titan and Triton -- the bodies that would arguably be planets were they circling the Sun instead of a planet. After that, there's the big jump down to Titania, Oberon and Rhea, which is a convenient place to draw the line.
However, as we're now seeing with the Pluto debate, defining things in terms of "convenient gaps" is a Very Bad Idea, because new discoveries tend to fill up those gaps. So, yeah, I could go for the "Big Nineteen" (plus whatever else turns out to be out there).
Posted by: djellison Aug 24 2006, 01:31 PM
Here's the big problem
There is, for whatever reason, a need to classify what is and isn't a planet. There is a body of support to ensure that Pluto remains within that group. However - any scientific set of rules that dictate Pluto is a planet will innevitably lead to the number of planets reach double figures and far beyond over the next few years - and that would dilute the 'power' of the title 'planet' The body of public support which would like Pluto to be a planet is self defeating in that regard.
ANY definition requires, at some juncture, numerical limits.
If you say 'if it's round' - how round is round? A radius that varies by less than 10% from the average? At some point you'd have to have a cut off between round and not round, and that seems quite arbritrary to me.
If two bodys orbit around a point outside the surface of the parent body, then the moon becomes a planet. That's not helpefull as moons can move. Charon could be a moon were its orbit a little different, and our moon could be a planet in a few billion years. Again - that can not be right because a cow is a cow regardless of WHERE it is. It doesnt stop being a cow and turn into a goat if you put it in a barn.
There seems a reluctance to extend the title planet to things beyond Pluto, as if they don't 'belong' there. A cow is a cow if it's in a field or on a pavement...and that rule should apply to a planet.
I'm quickly reaching the conclusion that NO set of rules will EVER be acceptable to a majority...there is too much culture, too much fondness and too much historical influence to ever make a decision that sticks.
Doug
Posted by: Stu Aug 24 2006, 01:35 PM
Just had this email, not sure if it's right or not...
"Leading astronomers have declared that Pluto is no longer a planet in approving
new guidelines that downsize the solar system from nine planets to eight, The
Associated Press reports."
Posted by: Greg Hullender Aug 24 2006, 01:51 PM
It's always nice when you can draw clear lines or find "convenient gaps," but for lots of real-world problems you just can't -- even though human beings confidently say "I know one when I see one." Computer handwriting recognition (for example) has this same problem, but in a different space.
I really think the IAU has reached the right decision, but I still don't think it's possible to make a principled definition without using statistics. You need at least a 2-dimensional (mass and semi-major axis) and probably 4-dimensional (eccentricity and inclination) model to capture what we intuitively see: e.g. the asteroid belt is a "fuzzy torus." So is the Kuiper Belt. Something that's really far off on any of those 4 parameters is NOT a member. A statistical model just lets you pin down exactly what "really far off" means.
This also lets you give a crisp definition to "cleared out it's orbit." The reason the trojan asteroids can't compete with Jupiter is that they clearly don't belong to the same distribution. Let's say you plotted the distribution (just by mass) of all the objects in Jupiter's orbit. There will be some number of small objects, more smaller ones, even more tiny ones, etc. I haven't seen the data, but I'd guess it's a Zipf distribution. From this model, you can predict how likely a new object (just observed) is to belong to the distribution. Jupiter itself, however, does not belong -- it's way, way too big. The math will give it something like 10^-1000 (or less) probability of belonging.
Ceres, on the other hand, fits quite nicely into its distribution. The largest members are close in size, and for any given range of sizes, (e.g. +/- 1km), the smaller the size, the more members there are. From a classification standpoint, everything in the Asteroid Belt is an asteroid.
The reason to want to include eccentricity and inclination is that prevents a passing comet from "becoming" an asteroid while it's passing through the Belt.
The eight planets are the big pieces that are left over after you've classified all the Asteroids and KBOs. Each planet is unique. They don't fit any of the other distributions, and they don't belong to one themselves. Unlike what Volcanopele said earlier, even if (say) Earth had the same orbit as Hally's Comet, we'd still call it a planet, if our definition is "orbiting the sun, large enough to be round and belonging to no other distribution of objects."
A different way to look at it is to say that we can give a good definition for things that are NOT planets; planets are whatever is left over.
This really will work in any other solar system, I think. I'm surprised not to hear other arguments from statistics, though. I know that statistical classification has been used to automatically find galaxy images in digital pictures, so Astronomers do know the technique, and they're not afraid of math in general. Also, althogh it can be tedious to characterize a distribution, for things like Asteroids and KBO's, you only have to do it once (for each belt). Orbital elements, by contrast, have to be computed for each individual object.
In sum, it seems that the neatest way to characterize a solar system (not just ours) would be to say, "It contains these planets, with these orbital elements, and these asteroid belts, with these distributions, plus an assortment of irregular objects." Given the orbital elements of any unknown object, you can instantly assign it to one of those categories, with some probability. Given the "sharp edges" in the solar system, though, I expect the probabilites for real objects to be close to 100% (probability that Juno is an asteroid) or close to 0% (probability that Jupiter is an asteroid).
Posted by: Jyril Aug 24 2006, 01:56 PM
Resolutions 5A passed and resolution 5B not passed, meaning Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune are planets and Pluto and other would-be planets are "dwarf planets".
Counting on Resolution 6A votes are under way, seems to be near-tie.
Posted by: Holder of the Two Leashes Aug 24 2006, 01:56 PM
In addition to the AP, there is now "breaking news" on Space dot com that Pluto has been officially demoted.
The working definition was round due to the fact that any rigid body forces were overcome by gravitational ones. The only significate out of roundness of such a defined planet would be due entirely to rotational forces or tidal ones, its shape still determined by hydrostatics.
If the moon Titan were ejected somehow from the Saturn system, and continued to orbit the sun, we would no longer be calling it a moon, and would most likely be calling it a planet. Orbital dynamics determine what is and is not a moon, and as the dynamics change, there is no reason why the definitions can't change, at least in my humble opinion.
Posted by: Jyril Aug 24 2006, 01:59 PM
I knew I can count on astronomers! Exactly the resolution what I wanted (i.e. Pluto not a planet nor a minor planet, er, small solar system object).
Posted by: Jyril Aug 24 2006, 02:02 PM
Resolution 6A passed, Pluto and other large trans-Neptunians form a new class.
Posted by: Alan Stern Aug 24 2006, 02:02 PM
Read 5A carefully. Earth is not a planet now, thanks to NEOs. Ditto Jupiter owing to Trojans.
Such a farce.
Posted by: Jyril Aug 24 2006, 02:08 PM
Such a farce.
"Cleared its path" = dominant object (why couldn't they keep that wording?) Therefore Earth and Jupiter are planets.
Posted by: djellison Aug 24 2006, 02:08 PM
Define significant. One could argue that the depths of the Pacific trenches to the peaks of the Himalaya would classify the Earth as significantly non-round. At some point you have to put a measurable factor into when something is no longer round. Is Phobos round? Mars? Olympus Mons + Valles Marineris makes mars quite un-round. I understand that the shape is determined by it's own gravity - that's fine....but you have to put down a marker to say at what point that shape is round.
That's a good point. So Luna, Titan, the Galileo 4....should all be planets?
Doug
Posted by: Alan Stern Aug 24 2006, 02:11 PM
Clared its zone is simple english. It doesn't say dominate naything. It says cleared. Earth is a dwarf. So is
Jupiter.
Alan
Posted by: ugordan Aug 24 2006, 02:13 PM
Well, according to the current definition if they were ejected from their planetary system and if they, by some miracle, achieved an orbit significantly different than their parent planet (the clearing out rule), it would be logical to call them planets as well.
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