Experts meet to decide Pluto fate, Finally we'll know what a 'planet' is... Options

[MizarKey]

One of many articles regarding the upcoming conference...

Experts meet to decide Pluto fate

[paxdan]

IMHO pluto is NOT a planet....

Just thought i'd kick off the inevitable debate

[akuo]

I'm going to start a campaign to remove the planetary status of Mercury, if they drop Pluto. It's a glorified Vulcanoid!

[David]

I'm going to start a campaign to remove the planetary status of Mercury, if they drop Pluto. It's a glorified Vulcanoid!

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...

[Ames]

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...

Ok, Jupiter is a failed sub brown dwarf...

Nick

[djellison]

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

[ups]

"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.

[rogelio]

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?

[JRehling]

[...]

[remcook]

JRehling, that's one of the most sensible arguments I've heard in this eternal discussion :-)

[David]

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.

[volcanopele]

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.

[rogelio]

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.

[Guest_DonPMitchell_*]

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.

[SigurRosFan]

In the news ...

Pluto the Ninth, Xena (2003 UB313) the
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.

[Alan Stern]

[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

[Greg Hullender]

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.

[ljk4-1]

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

[MahFL]

I would like Pluto to remain a planet.

[ngunn]

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.

[volcanopele]

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.

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...

[Guest_AlexBlackwell_*]

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.

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.

[David]

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.)

You're right, there wasn't. The following historical summary discusses how it happened:
When Did the Asteroids Become Minor Planets?
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.

[volcanopele]

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.

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.

[David]

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.

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?

[Alan Stern]

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?

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

[David]

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:

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?

[Alan Stern]

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?

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.

[David]

The roundness argument is not about whether an object is round or not -- because it could for example be tidally bulged or rotationally distorted. It's about whether it's massive enough **to be rounded by gravity** in the absence of the other effects.

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!

ps. EL61 is probably not a big egg: it's most likely a huge contact binary. At least that's where my money is.

Interesting thought.

And for the sake of amusement, the author of a "liveblog" from Prague announces that:

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.

[Alan Stern]

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 "liveblog" from Prague announces that:

Let's discuss this tomorrow after you see what the IAU position is. Some of your questions will be addressed.

-Alan

[ljk4-1]

And for the sake of amusement, the author of a "liveblog" from Prague announces that:

"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.

[JRehling]

[...]

[Guest_AlexBlackwell_*]

I quite agree -- my question would be: why take the old, problematic term "planet" and try to shoehorn it into a nice, logical taxonomic system? It is precisely because the term is in the popular lingo that it's ill-suited for such a role. To me, trying to tinker with the popular term "planet", changing it, in order to get a useful taxonomy would be like trying to come up with a geological definition of "hill" as opposed to "mountain". Because there once seemed to be a sharp divide between planets and asteroids, the usefulness of the term was unquestioned. Now that the divide is known not to be sharp, the question is: why mold the term instead of working aroundit? People still have their nonscientific words for mountains and hills, and it doesn't hurt geology.

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 Alan Stern et al. ever find the hypothesized population of Vulcanoids, then I have no sentimental attachment in reclassifying Mercury, as well.

This post has been edited by AlexBlackwell: Aug 15 2006, 09:06 PM

[DFinfrock]

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.

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

[volcanopele]

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

Obviously they mean the absolute visual magnitude of the body itself, not any associated coma or other debris cloud.

[dvandorn]

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

[Holder of the Tw...]

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.

[mcaplinger]

They report that there are twelve known planets in our solar system under this definition.

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.

[volcanopele]

? 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...

[dvandorn]

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

[nprev]

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".

[volcanopele]

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.

[Guest_DonPMitchell_*]

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?

[djellison]

Totally unrelated politics and political imagery removed. You all know the rules guys.

Doug

[MichaelT]

The relevant IAU press release can be found here:
http://www.iau2006.org/mirror/www.iau.org/...resolution.html

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.

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

[ngunn]

I wonder what happens if the mutual orbits are eccentric and the barycentre moves in and out of the larger body every 'month'?

[djellison]

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

[ugordan]

I wonder what happens if the mutual orbits are eccentric and the barycentre moves in and out of the larger body every 'month'?

Perhaps an "average" barycenter point can be taken as measure in that case. Say circularizing the orbits, leaving the orbital periods constant?

[paxdan]

PLANET SIZED

What the hell does that mean

Pah! that's an easy one: 950 to 142984 km.

[Guest_JamesFox_*]

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.

[David]

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.

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).

[Guest_JamesFox_*]

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".

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.

[David]

The problem with this is that the sizes of many objects are currently very uncertain.

That's why God invented the asterisk.

[Ames]

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.

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

[Ames]

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

Actually Barry Senter

www.barrysenterdesign.com

Hmmm!

Nick

[David]

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"?

[MichaelT]

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"?

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

[ugordan]

So they are two planets revolving around each other.

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?

[paxdan]

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?

Barry Sentor would like a word

[ngunn]

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

How about: Many Vexed Experts Make Confusing Judgment So Us Normal People Say 'XXXX'

[ljk4-1]

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.

[rogelio]

Dateline 2015: U.S. Postal Service issues revised planetary exploration postage stamp series ending with “Xena –Not Yet Explored (or formally named)”...

[Ames]

How about: Many Vexed Experts Make Confusing Judgment So Us Normal People Say 'XXXX'

I like it

Nick

[Alan Stern]

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?

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.

[ngunn]

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.

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?

[maycm]

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

[ugordan]

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.

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.

[David]

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.

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.

[djellison]

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

[Tom Tamlyn]

Today's New York Times has a good article on the controversy, with quotes from Alan Stern, as well as an op-ed piece by Mike Brown.

TTT

[David]

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.

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..."

[ngunn]

I like it

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.

[Greg Hullender]

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.

[mcaplinger]

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.

[ljk4-1]

A camel: A horse designed by committee.

[alan]

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.

[David]

I'd have tried to make the minimum barycenter distance some function of the body radii so as to exclude Charon.

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.

[JRehling]

[...]

[jsheff]

Results are in:

IAU

[maycm]

Results are in:

IAU

....well not yet.... they vote Thursday 24th August

[Guest_AlexBlackwell_*]

I'm not sure if this has been mentioned, but here's Kevin Drum's take on the issue.

[vexgizmo]

"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?

[David]

I don't see how, especially with distant KBOs and extrasolar objects being up for consideration, anyone can be happy with a criterion that depends upon precise measurement. This would mean that as new observations are made, we'll discover planets, then, with arbitrarily minute revisions, have to say in some cases, "We were mistaken -- that wasn't a planet."

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.

"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...."
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.

[hendric]

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.

[Guest_AlexBlackwell_*]

Ultimately all taxonomic systems have "marginal cases" problems. That's not a flaw in the concept of taxonomy, it just comes with the territory.

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 oral argument in Roper v. Simmons:

...but the purpose of a [constitutional] bright line test is to avoid litigation over the borderline cases...

[Guest_AlexBlackwell_*]

The editorial that appears in the August 17, 2006, issue of Nature:

Editorial

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.

[JRehling]

[...]

[Alan Stern]

[
> 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

[vexgizmo]

Alan Stern discussed this above (post #28 above, with my reply following).

I's about whether its massive enough **Tto be rounded by gravity** in the absence of
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.

[David]

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 Xena doesn't seem to make the cut until we measure its triaxial shape.

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.

[Phil Stooke]

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

[JRehling]

[...]

[Guest_AlexBlackwell_*]

Phil Plait has an interesting take on it.

[David]

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.

Is that: if the maximum is 110% of the minimum? Or: if the minimum is 90% of the maximum?

[Guest_AlexBlackwell_*]

Alex, I'm trying to parse down to the nugget of your reasoning there. Because the term "planet" has changed considerably since its origin, it's OK to change it again now?

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:

So 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?

[volcanopele]

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?

[JRehling]

[...]

[David]

And, as I mentioned before, what about a very planetlike body (Earth, Saturn -- take your pick) that is in deep space, not orbiting a star. If it's not a planet, what the heck is it? Or if it were orbiting an object that is not a star (a pulsar).

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.

[Jyril]

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).

[jsheff]

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