The new definition has something amusing. Way back when on the Spacebattles forums, a couple of ST-v-SW.Net's loyal opponents were shown a video clip of an exploding 'planetoid' in a Voyager episode. Screenshots from the video are still here.
Their claims were simply astonishing. The round, partially cratered body was said to have dilithium deposits "beneath the planetoid's crust [...] in the upper mantle, about sixty kilometres down". However, the opponents opened by arguing that the planetoid was a mere two kilometers in size.
Besides the depth given in the dialog, the mention of differentiation (i.e. crust, mantle, etc.) implying a certain size, the visible evidence of geologic activity, and so on, one of the points I made was that since the planetoid was spherical, it almost certainly had to be hundreds of kilometers in size because of the gravity required, based on our own local solar system bodies. This logic was rejected by the opposition, but eventually it was claimed that the roundness may have been artificial.
The IAU just gave me the authority to whip out a heaping handful of I-told-you-so. I quote:
"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 [...] his generally applies to objects with mass above 5 x 1020 kg and diameter greater than 800 km."
But anyway, the IAU definition attempt is bound to be controversial. While I'm a sentimental fan of Pluto being kept as a planet, the inclusion of Ceres will undoubtedly confuse folks.
But really that's just a "re-inclusion", since it was considered a planet when first found. People were actually looking for a planet in that spot when they found Ceres, since Ceres (and the rest of the asteroid belt) satisfies the Titius-Bode Law.
Also at issue is that there may be dozens of other valid contenders. As it stands, the IAU definition will, at present, bring the number of planets to 12. One of those twelve is Charon, long considered Pluto's moon. I still remember a National Geographic planet poster I had as a kid that said "small as it is, Pluto has a moon" ... but now this will be identified as a double-planet system since Charon qualifies independently, and the two orbit one another around a point not occupied by either.
But another dozen are on a "watch list" for planethood. Some are other asteroids like Ceres but smaller, whereas others are more of the same iceballs from the Kuiper Belt and Oort Cloud areas like Pluto or Sedna.
Though it might seem arbitrary, I would think a population criteria might be of value. The original reason Ceres stopped being called a planet is because other asteroids were found in the same orbit. The term "planet" comes from the Greek for "wanderer", and all eight classical planets share the fact that they wander alone. Ceres is part of a tribe of wandering bodies, some coming close to its size. Pluto and other iceballs wander about the outer solar system rather drunkenly, with highly eccentric orbits and so on.
Opposed as I might be to Pluto's demotion sentimentally, I can see the appeal of it rationally. But instead, it looks like Pluto's going to have a lot of company.
But I suppose we'll find out when the draft definition gets voted on later this month.