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.

Except for Mercury...

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.

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.

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.

Why is demoting Pluto such a big deal? There were lots of such cases - bodies were demoted as our knowledge grew:

List of demoted planets in the Solar System

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?):

Dava Sobel "Making" New Planets

And it is REALLY interesting to know that Dava Sobel is author of "The Planets" 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:

Dava Sobel says ASTROLOGY is good

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.

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!

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.



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.

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.

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.

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.

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.

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 ;-)

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.

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.