Scott, what I actually said was, "None of the eight classical planets fully clears its orbit" (emphasis on the word "fully"). This goes back to the question of what criteria should we use to determine if a neighborhood is "cleared?" All of the eight classical planets have asteroids within their orbital fields, and Neptune does not "clear" its orbit of Pluto. So if this criterion is applied in a very literal sense, one could say none of the eight fully clear their orbits.

Dan, the planet definition issue first came up not in 2006 or 1999, but in 1992, when the first Kuiper Belt Object was discovered. This is why Tombaugh, who died in 1997, was already aware of the controversy (although he always maintained that Pluto is a planet).

" A few raging bullies?" Are we seeing your true feelings about Dr. Stern and planetary scientists who disagree with you and with four percent of the IAU? It seems there is a bit of the pot calling the kettle black here, as it is that four percent of the IAU who are attempting to impose their definition on the rest of the world. It seems to me you're the one who has started the name calling–"Pluto lovers," "raging bullies," accusations of "shouting," etc. If you are incapable of conducting a mature debate with those of differing opinions, please do not blame others for your shortcomings.

It's all in the definition of "clearing". If you define it as meaning that you can't have anything nearby heavier than, say, 10 tons, then there's not a single planet in the Solar System with a clear orbital path. If you define it as meaning that you can't have more than the object's worth of mass in its path, (and define the path as being a more or less wide volume of space, of course) then only the 4 gas giants and the 4 terrestrials make the cut. And which of these definitions is the correct one? Since where you put your limit is totally arbitrary, they both are.

This said, I must say that I don't like this argument. It's useful to show how the current IAU definition is based on arbitrarily chosen values, but it's not a good argument to suggest it doesn't work. Because it does work, for the solar system as it currently stands, provided your definitions of "clearing" and "neighborhood" are carefully selected.

The dynamic approach has other weaknesses, much more serious than that one, and I think it's wrong to focus on something that can be easily rebated or explained while there are other arguments that dynamicists have a much harder time with. I spread a list of these throughout my comments.

Good night and good luck …

And do you really want an honest reply as to why few people bothered with this before it became an issue? Not with that IAU nonsense, but with the discovery of Eris?

Of course not: you have no interest in really discussing this, otherwise you wouldn't go around twisting what people write, putting falsehoods in people's fingers and being a brat overall. Your only interest is to try to be right at any cost. And that, kid, is wasting everyone's time.

Still, as I don't plan on wasting my time with you any longer, I'll explain this just this once.

See, before the discovery of Eris practically nobody really cared to open the can of worms that is the definition of planet. Things didn't make much sense, true, but, hey, Pluto was the largest of the TNOs, nobody really knew how round were the other TNOs that were being discovered since the early 90's, and it wasn't until the Hubble imaged it in 2003 that we found out (confirming some previous suspicions) that Ceres showed indeed planetary characteristics. So everything was kinda fine. The peace showed signs of rot, but it was a kind of peace.

But then Eris came along. And with its arrival, the can of worms had to be opened. Lots of silly things were said and written at the time, both by the "pluto-lovers" and by the "pluto-haters", but more normal people thought "screw Pluto; let's find a definition that makes scientific sense, and then we'll see what happens to Pluto." Unfortunately, not only there's a big scarcity of normal people in this planet, they also split in two sides, rapidly overwhelmed by the silly types: those that thought that orbits were more important, and those that thought that the physical characteristics of the bodies themselves should take precedence. The commission that prepared the original IAU proposal was composed by normal people of this second flavour, but they also fell pray to sillyness with that pretty inept double planet thing. And the rest, as they say, is history. History that was so silly, that passed along such a nutty image of the astronomical community that most of the normal people are now in the position of "screw all this, let me just get back to work".

Which is what I'm now going to do.

(regarding the claim you make on the 1-2 Earth masses limit, I wasn't aware of it but, well, I must say that I'll believe it if and when I find it written out in a credible outlet. After what you've shown us here, I'm afraid I'm taking everything you write with a very heavy grain of salt. Probably heavy to the point that its self-gravity overcomes rigid body forces)

In response to PaulS's statement, "But of course, the IAU gets to decide on the definition, and we can only play at definitions," this may be changing. The IAU has lost a tremendous amount of credibility in this debacle. No electronic voting, backroom deals where even leading planetary scientists are not informed of the discussion (the plutoid decision), a sloppy definition–all have generated a movement among many astronomers and planetary scientists to either expand the number of those making these decisions or form an alternate group that can conduct a better process. Any organization that makes a decision leaving out 96 percent of its membership has major problems.

Dan, you may have seen the 12 planet proposal the day before it went public, but clearly a committee had been working on this for some time. That committee included Ceres as a potential planet because of its known state of hydrostatic equilibrium. The entire issue of planet definition never was just about the Kuiper Belt. It was always about what criteria made something a planet, and once Ceres' roundness was known, its candidacy for planet status came up for discussion any time this debate was taken up.

It appears you will not share the names or comments of astronomers you claim signed Stern's petition "for totally non-scientific reasons." Why this sudden need for privacy? You're claiming these scientists signed their name to a petition supporting a principle in which they do not believe. That amounts to questioning their integrity and honesty. If you're a journalist, why would they tell you they didn't believe in what they signed and not expect their statements to be made public? Quoting sources is what journalists do. And why would they tell you and only you one thing while publicly signing their names to something contradictory? Without specific names, dates, and comments, your claims hold no validity whatsoever.

You state, "You simply cannot take a concept ingrained in human history and redefine it radically to your liking." Yet that is exactly what the IAU did. The definition they created makes no scientific sense whatsoever for multiple reasons including the claim that a dwarf planet is not a planet and the requirement that to be a planet an object clear the neighborhood of its orbit. Applied literally the latter statement could be said to exclude all planets in our solar system. And if Earth orbited in the Kuiper Belt, it would NOT clear its orbit of other KBOs, meaning according to the IAU definition, it would not be a planet. In the end, the IAU definition is untenable because it defines objects solely by where they are, not by what they are.

I have no problem including moons in hydrostatic equilibrium as secondary or satellite planets. This in no way makes them "inferior" to primary planets, which orbit stars. It simply takes into account the orbital criteria that dynamicists value so highly.

The umbrella idea does not establish a hierarchy, simply a list of subcategories. Dwarf planets are in no way any lesser or inferior than terrestrial planets–they are simply in a different subcategory.

It seems like you're the one shouting with your ad hominem attacks on "Pluto lovers." How do you know whether or not anyone was thinking of geophysical arguments before 2006? Opponents of Pluto's demotion take the stand we do because the decision was made in a hurried, backroom deal, exclusionary process and because the "definition" the IAU came up with is terrible. Is the term planet special? Yes, of course. So are the terms star and galaxy. That does not mean there can not be billions of each (with many subcategories). Those of us who support Pluto's planethood do so because we advocate a broad definition of the term planet to mean any object in hydrostatic equilibrium orbiting a star rather than an artificially narrow one. Additionally, I also see no problem with the number of planets in the solar system being left indefinite, or in flux, as more are obviously being discovered out in the Kuiper Belt. Why set an artificial limit?

And ask yourself why the Pluto-lovers were so upset by the alleged 'demotion' of their buddy. Because it was no longer belonging to that special group of bodies circling the Sun. Because – like most people – they felt that a planet was not just a round thing orbiting a star but something more important, outstanding in a solar system.

For if that would not the case, why all the anger? Pluto would have been the smallest and oddest of nine planets, now its one of only four in a class and even the first one found: actually a steep upgrade. But no, the Pluto-lovers rather throw around physical arguments they wouldn't have dreamt about before Aug. 2006 – or why weren't they fighting for Ceres' honor at all before '06?

Now just assume that the umbrella idea would have been accepted, with an indeterminate number of planets, divided into major/main/classical/real planets, dwarf planets/planetinos/… and minor planets/asteroids. Still Pluto would have ended up in the middle faction unless a very artificial argument would have kept it a major planet (like that "it was one for 60 years" …).

Would that have been "better" than the current division of planets – dwarf planets – minor planets? Or would it have led to the same fights we have today? To use the chordate analogy: The 8 real planets would certainly be (importance-wise) the vertebrata, the dwarf planets the tunicata and the minor planets the cephalochordata. Now don't tell me that the Pluto-lovers would be happy with their non-vertebrate baby …

And in case you're wondering what's intellectually dishonest about what you wrote (as if you didn't know), I point to one instance, among several others I could have chosen. This one: Jorge can't make up hismind what to do with the large satellites, so he arbitrarily leaves them in a lesser category.

Apart from intellectual dishonesty, there's another adecuate name for this sentence: BS.

You may not understand the difference between "not being able to make one's mind" and "putting forth an alternative". I, and many others, do. I happen to know what I prefer, but I also happen to know that (how did you put it?) "you simply cannot take a concept ingrained in human history and redefine it radically to your liking". I happen to prefer to take into consideration all the comings and goings of planetary objects and call "planet" to everything in hydrostatic equilibrium. But I know that most people will reject as violently the idea of calling "planet" to big satellites as they did reject the whole "Pluto-Charon as double planet" thing (which was indeed the weakest link in the original IAU proposal), so I'm pragmatically willing to support an alternative that demands that planets should orbit stars. Not the two at the same time, though. Only someone who believes that something called dwarf planet is not a planet would believe that those were supposed to go together.

Oh, wait…

And only someone who thinks that a subcategory of planet is something inferior to a planet can come up with that beautiful notion that saying that something is a satellite planet is labelling it as "inferior". That's just as smart as saying that a mamalian chordate is an inferior form of chordate.

Go check your rudiments of set theory, Dan. You probably also learned something of the sort in school. Go see what's "inferior" about being a member of a subset of a larger set. I'll wait.

This, now, is quite revealing: Leaving the number of planets either highly indetermined or near infinity was not an option. It's the "500 freaking planets" factor all over again. It is completely un-scientific, which makes any argument about scientificity in people who think that way simply loughable. And derives from a fundamentally wrong notion of what the cultural idea of planet is. Ask anybody to describe a planet, and you'll get something like "a round thing orbiting a star", not "one of 8 things around the sun". When you ask what a planet is, people don't come up with a number; they come up with a shape. You start getting numbers when you ask stuff like "how many are they?"

And no, I won't repeat everything I wrote about extrasolar planets. I know you understood what I wrote fairly well, but, again, you're just being intelectually dishonest ignoring what doesn't suit your agenda and setting forth false and deceptive information, like that radial velocity curves are "extremely well-defined". Do yourself a favor and go see the error associated with the best fit solutions of radial velocity curves, partly due to instrumental error here on Earth, partly due to stuff instrinsic to the systems, like stellar jitter. They are far better than a few years ago, but the fact that they are still not finding anything below some 8 Earth masses should tell you something about their sensitivity.

And besides I still haven't found anything even remotely non arbitrary to tell me what the heck a "neighborhood" is. Take the HD 69830 system, with its three neptune-sized planets packed in 0.63 AU. They're all influencing eachother's orbits, perturbing them to a major extent. Not those puny perturbations we have around here, out there it's major. If a neighborhood is a zone of influence (and that derives logically from the notion that a planet should clean it), then those three planets are all squarely in eachother's zones of influence. So what's there should be considered a belt of neptunes, and therefore those three neptunes should be called "dwarf planets". Which is a nonsense, of course.

Ceres' neighborhood, on the other hand, is the whole asteroid belt. It goes from some 1.7 to about 4 AU, but let's just think of its core. It ranges between 2.2 and 3.2 AU and probably contains almost all its mass. Ceres' orbit, however oscillates between 2.545 and 2.987 AU. There's plenty of asteroid belt quite far away from Ceres' orbit (0.345 AU to the inside – more than the distance between Venus and the Earth, 0.213 AU to the outside), and yet it's in its "neighborhood". Why? Just because.

Yeah, right.

Finally, the Oort cloud. Dan, nobody knows how much mass there is in the Oort cloud. I have seen estimates from about 5 to about 400 Earth masses. Let's assume it's just 5. Let's assume 1 Earth mass of that is in a planet. The volume of space there is so incredibly vast that it would never have "cleared its neighbourhood" of the other 4 Earth masses. Not by a long shot. Because, among other reasons, by the same reasoning that puts all the asteroid belt over tthe poor shoulders of our pal Ceres, all the Oort cloud would be on the "neighbourhood" of that earth sized planet (which no, is not Niburu). Which would then have to be called "dwarf-planet-not-planet" by you people. An Earth-massed planet, larger than the Earth because it would be composed of large quantities of ice and therefore would be a lot less dense than our planet is, and yet no planet.

Which is just plain crazy.

Both he and PaulS – and many other critics of the status quo – also fail to see what the whole IAU business was about in the first place: coming up with the first ever definition of a planet that made sense scientifically (almost done , made cataloging and naming rules unique (done) and respected cultural traditions of centuries (done).

Leaving the number of planets either highly indetermined or near infinity was not an option (although some in the original definition committee would have accepted the former – their greatest blunder): You simply cannot take a concept ingrained in human history and redefine it radically to your liking ("historical definitions don't matter"), even if it made sense per se.

Finally, about the exoplanet issue (that was not on the IAU's agenda in 2006 but may well be in 2009 – oh dear …): The situation is probably clearer than Jorge wants to admit. In our solar system there are either planets with very little stuff in the vicinity (NEOs for Earth, Trojans for some others, the rogue Centaur etc.) and dwarf planets embedded in belts of minor bodies that in their sum have more mass.

Even if you just have a radial velocity curve (and these have gotten extremely well-defined these days, thanks to hi-res spectrographs like HARPS), how would you hide other bodies of a combined mass greater than the disturbing body in the same orbital neighborhood with influencing its orbit in a strong way? Let alone make such a system dynamically stable over long time?

Finally to answer a direct question from Jorge: Eris in the inner solar system? A planet, of course (if it wouldn't sublimate too strongly . Earth way beyond the Kuiper Belt? A planet, too, since the Oort cloud is extremely diluted, so Earth's vicinity would be "clear" enough anyway under any reasonable detailled definition of clearing.

EL61, Sedna, and a number of other TNOs "are not planets on my list" for the exact same reason they are not dwarf planets in IAU's lists: we still don't know if they are really massive enough for their gravity to overcome rigit body forces. You may not know this, but masses of TNOs are inferred from their apparent magnitude, except when moons are found, whose orbits allow those masses to be directly measured. And the apparent magnitude method is highly dependent on guesses regarding surface characteristics, hence albedos, and subsurface composition, hence densities. There's some spectrographic information, but it's far from enough to say anything for sure. We simply don't know yet if these bodies are massive enough to be planets or not. Most of them probably are, but until there's some degree of certainty they won't make the lists. Because they are not assigned arbitrarily to a category.

When you understand this, you can cut 80% from those 100% of yours.

Titan and the Jovian moons are not planets because they are… surprise… moons. Wow! This one was hard. But, since you apparently didn't read what I wrote, let me repeat it: "If you want to accomodate Dr. Heinke's argument about the possibility of capture and release of moons, you may add to [the 12 planets] some two dozens of "satellite planets". Patiently explaining better: if you want to accomodate that argument, then Titan, not the Jovian moons but the Galilean ones, our own moon, a bunch of moons of Saturn, another bunch of moons of Uranus, Triton (which seems to have been indeed a planet in the past) and Charon would also constitute another class of planets, the satellite planets or whatever you migh want to call them. Else, they wouldn't. If, then, else. A conditional expression. You might have been taught how those work at some point during your early school years.

When you understand this, you can cut another 19% from your remaining 20%.

There's a 1% arbitraryness left dealing with the fact that different compositions have different hydrostatic properties, and that no planet with a solid surface is in absolute hydrostatic equilibrium. I can live with that.

(these percentages are, of course, to be taken just as seriously as Dan's 100%, i.e., total bogus)

With respect to the exoplanets, you don't seem to realize just how big the observational error still is in anything that is not transiting. The orbits of transiting planets can be characterized fairly accurately, especially if you're dealing with hot jupiters (and you usually are), but that's not so with the other methods, which produced the vast majority of the exoplanets found so far. Some periodograms don't even have a single solution yet. There is no way to detect earth-sized orbit crossers, especially if they're in resonant orbits, and won't be for a long time; they're harder to detect than Earths in "normal" orbits, you know?, and we haven't even found these (although we're close).

Mind you: I don't doubt for a moment that the vast majority, if not all, of the exoplanets found so far have "cleared their orbits". After all, they are all gas giants or super-earths, most in close orbits around their stars, and one of the biggest flaws in that whole orbit cleaning thing is that it's much faster near the star than far from it. Put an Eris-sized body at 0.05 AU around the Sun and you'd have to call it planet even by your beloved IAU definition, because it would most certainly have cleared its neighborhood (unless you decided arbitrarily that its "neighborhood" included Mercury, of course). Put an Earth sized body in the Oort cloud and it probably wouldn't, even today, after 4.6 billion years of "merry-go-rounding".

But I also have no doubt at all that the smaller the detected planets are, the harder will be to know if there's anything at all sharing their "neighborhoods". It'll probably take us a couple of decades to be sure about the non-transiting gas giants, perhaps less than that for the first ones (and the ones in more compact systems) but when the talk shifts to super-earths and earth-sized planets, we'd be looking at much longer periods of not knowing if a given planet has cleared its neighborhood or not. I'm talking about centuries here.

Putting that stuff in the trash can where it belongs as far as defining planets goes, we can say with absolute certainty now that all the extrasolar planets detected so far are indeed planets.

Simplicity.

I'm having fun here … just so you know, I'm not proposing that the IAU change their designations, just explaining that I would have chosen a different criteria.

What does it matter what whether anyone included comets as a planet before? With my criteria, size doesn't matter, quantity doesn't matter, historical definitions don't matter. What matters is whether the body orbits a star or not.

Meteors are meteors, not planets, because they have entered a planet's atmosphere and are no longer orbiting a star. But a meteoroid … that's a planet unless it has left the orbit of a star. Then it becomes an interstellar body.

And w.r.t. the exoplanets: Thanks to – the absence of – orbital deviations we know (or will soon know) that there are no other bodies of even remotely similar mass in their orbits: they are, gasp, clear(ed)!

Thus the definition – not even meant for exoworld yet – actually works there well. Perhaps the IAU voters were even wiser than they realized back then …?

@PaulS: Counting comets as planets would be so stunningly un-historical that no one has, to my knowledge, ever brought up that idea. Planets were equated to gods, comets were not even astronomical phenomena to the ancients (and well into the past millennium in some cultures).

You also didn't address the issue of a lower limit. Meteors, in your definition taken to the extreme, would be planets (having been in solar orbit for some time) burning up in our atmosphere. Ouch …

@Laurel: Show me one scientific paper or popular article between, say, 1900 and 2006 August 16 that called for Ceres becoming a full planet again. All the debate in the last 15 years in particular had been focused on the Kuiper Belt when bodies with 1/3, 1/2, 2/3 and eventually more than Pluto's size were discovered. The main belt was never touched and always considered an agglomeration of planetesimals that Jupiter prevented from forming a planet.

To clarify my "role" in the IAU process, again: I was a volunteer editor with the GA newspaper and thus saw the 12-planet proposal 2 days before it went public. (It also leaked to Czech TV but no one noted that.) I was present during the crucial – non-televised but fully public – debate on August 18 that brought the 12-planet idea to a sudden death and forced a major rethink. Being a journalist, I didn't say anything but took notes and videotaped some of the fascinating exchanges; one member of the planet definition committee (the folks with the 12 planets) asked me for my opinion, though. And another member conceded to me that the gravitational dominance criterion would have been just as good.

When the so-called petition was out, I contacted several of the best-known astronomers on the list of signatories to find out why they had signed a petition that was devoid of any argument for its case. To my honest surprise I found out that those particular astronomers more often than not had personal, totally non-scientific reasons. The collected responses were sent to the IAU President who found them "interesting" – perhaps she'll share them with you. I will certainly honor the privacy of the respondents.

@everyone: There is now an interesting speculation that Ceres is actually a Kuiperoid (and thus plutoid) that moved from this belt to the main belt in the past. Would explain neatly why it's so unusual (diameter- and mass-wise). But still no planet as it's not nearly massive enough the get rid of all its neighbors …

I didn't exclude comets from the definition. In fact I explicitly mentioned the Oort Cloud, home to billions, maybe trillions, of comets.

I guess where I was headed with my definition is that I feel the basic characteristic of any non-stellar body is its relationship to the object it orbits. If it is a star, the orbiting body is a planet (unless the body itself is a star). If the parent body is a planet, the orbiting body is a moon.

I feel the physical nature of the body doesn't necessarily delineate it from others. How is the Moon different from Mercury, other than size? Both are spheroidal rocky bodies. What is different is that Mecury orbits the Sun, while the Moon orbits Earth.

But of course, the IAU gets to decide on the definition, and we can only play at definitions.

"DavidV, you seem quite ignorant of the fact that Canada is in North America (the northern half) and we have been metric since the eighties!"

Thank you Tim for the update. I sincerely hope that you (The Canadians as a nation) will encourage your neighbours to adopt metric measurement and so ease international working.

I agree there is no perfect solution, but at the same time, emphasize that we should not let the perfect be the enemy of the good. It's not that I and others want the dwarf planet category to be an intermediate category between minor planets and real planets. What those of us arguing in favor of a geophysical definition seek is the classification of dwarf planets as "real planets," specifically as one subtype of "real planets."

Interesting that you cannot recall anyone asking for Ceres to be reclassified as a planet in 150 years, not just because I doubt you were around that long, but because the initial IAU 12-planet proposal did just that. Even when that failed, had resolution 5b passed, dwarf planets, including Ceres, would have been classified, as they should be, as a subcategory under the umbrella of planets (remember Dr. Burnell's umbrella, props, etc.?).

It's not clear whether 2003EL61 is or is not in hydrostatic equilibrium. However, the criterion of hydrostatic equilibrium is still the best measure of what an object is and what geophysical processes it has. For small objects this far away, we may need to reserve judgment on classification or create a temporary "uncertain" classification until we can better image them and determine whether or not they are in hydrostatic equilibrium.

The IAU placed all the pressure on itself during that week in 2006. There would have been nothing wrong with postponing the decision and taking more time to deliberate and come up with a better definition.

And how is it that the proposal was leaked to you before it went public, that astronomers spoke to you about which objects may be included, etc. It seems a lot of your arguments center around yourself and claims of your influence within the IAU. On another blog, you claimed you personally spoke with the 300 signatories of Dr. Alan Stern's petition rejecting the IAU demotion and claim they only signed for "political reasons." Please supply the names of the signatories with whom you spoke, the approximate time (month, year) you spoke with them, and the specific words they said. If you can't, these personal claims are no more than hearsay.

It's a shortsighted argument, dealing only with the current state of the Solar System (an insignificant corner of the universe by any measure), and forgetting not only that orbits change, even for what it now considers to merit planetary status, but also that there are billions of planets around other stars in all sorts of dynamic configurations. The universe is so vast, that it will almost certainly contain examples of anything that is possible by the laws of nature.

Let me quote part of one of the opinions present in the page you linked:

[...] recent discoveries of planets around other stars have emphasized the importance of dynamical alterations in planets' orbits in the final organization of extrasolar planetary systems. It is now believed that large numbers of (what might be considered) planets are ejected from extrasolar systems, due to interplanetary dynamics or to interstellar dynamics in clusters of stars. Free-floating planets have now been observed in the Orion Nebula. It is likely that some objects may at some times be satellites of other planets, and at other times circle their star by themselves.

Therefore I suggest that the most stable definition of a planet would be a physical definition, without a dynamical constraint. [...]

(Dr. Craig Heinke)

This is absolutely true, and the failure to understand this simple (IMHO) truth by the astronomic community at large is quite amazing. It's as if each astronomer were only able to see his/her own limited field of research, failing appalingly to even acknowledge that there might be a larger picture to this.

It's true that there is no perfect solution. That all definitions come with some degree of arbitraryness (although it's larger in some, and lesser in others). Yet there are solutions that are better and other solutions that are worst. The gravitational dominance argument is probably the worst possible argument that could have surfaced if you just raise your stare and look at other places, regions beyond our Kuiper Belt, and the possible past and future of our own system.

It's easy, really: scrap from the definition anything not dealing with hydrostatic equilibrium. You won't be "making everything going around the Sun a planet", but simply calling "planet" to Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Makemake and Eris, for the time being. If you want to accomodate Dr. Heinke's argument about the possibility of capture and release of moons, you may add to this some two dozens of "satellite planets". And you can safely call "planet" to the 200+ extrasolar bodies this IAU definition leaves in the limbo, but which are obviously planets just like our 12, without having to wait for a full knowledge about their systems that allow you to know if they have "cleaned their orbits" or not.

In any good classification system, simplicity is the key when establishing broad categories. That's why we chordates are the animals which, at some pont in their development, show a notocordium. Simple as that.

Then, subcategorize. Mammals are chordates with mamary glands. Belt planets are planets that have not cleaned their orbits. Birds are chordates with feathers. Dwarf planets are planets below a certain threshold radius.

Simplicity.

IMHO there simply was and is no perfect solution: Either the gap between planet and not-planet would have been set arbitrarily or most everything going around the Sun would have become some kind of planet. The introduction of the gravitational dominance argument – already in use then among some planetary scientists, by the way! – at least presented a fundamental way out.

If you make everything going around the Sun a planet (but exclude the comets? Oops?) and then try to define subclasses you still run into the same problems with a clear-cut dividing line. For example when is it an interstellar dust particle, when a meteoroid, when a minor planet? And now we would probably be fighting over what a "classical" planet is: The Sun and the Moon were planets in "classical" times, you know …

Of course, that would mean we have millions, possibly billions or even trillions of planets, since every body in the Kuiper Belt and Oort Cloud would qualify as a planet. But that's what sub-categories are for: dwarf planets, minor planets, etc. We could even have a "classic planets" that would include Pluto, for those who can't get over its demotion.

And indeed nothing is new, here. Jupiter itself is significantly un-round: there's a difference of about 4500 km between its equatorial and polar radiuses. That's about 2/3 of an Earth radius, in case anyone's wondering. And yet, it's obviously in hydrostatic equilibrium.

EL61's main problem is the struggle over priorities. Its shape is irrelevant as it is more than likely in hydrostatic equilibrium, despite its elongated shape. Once the priorities feud is solved and the IAU knows whom to ask for a name proposal, it'll be named and will become yet another planet of the dwarf subcategory. A bizarre planet, granted, but a planet nonetheless.

Let me take you back to August 2006, just before that bizarre "12 planets" proposal was made public (but had been leaked to me already): I asked a number of astronomers and planetary scientists if they could guess which 3 'new planets' that might include, and not one hit on either Ceres or Charon.

The hydrostatic equilibrium/roundness criterion – that the final definition for both planets and dwarf planets inherited from the defeated proposal – may already be back haunting us: Why did FY9 get a name and dwarf planet status but 2003 EL61, the third biggie announced in 2005, didn't?

This may be due to an ongoing struggle over priorities of its discovery but it should also play a role that this body, despite being as big as FY9, is decidedly un-round, thanks to its rapid rotation.

If you really want a dwarf planet category between the minor planets and the, ahem, real planets, I'm not so sure that the last word on the precise definition of their physical state has been spoken. Then again, no one asked for that category in the first place: I think we can agree on that if on little else …