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

[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