Pluto, a Planet

via Space.com

Well, the definition of planet has been announced and of course, there is some controversy. The definition laid out somewhat like what I envisioned, where something with enough mass that gravity causes a spheroid shape (hydrostatic equilibrium) and is in orbit around a star without being a star or a satellite, is a planet. The definition only addresses the delineation between planets and what are now referred to as "small Solar System bodies", the old term "minor planets" used to describe objects not a part of the old nine planets, is now being declared a defunct term.

"Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor," said Richard Binzel, an MIT planetary scientist who was part of a seven-member IAU committee that hashed out the proposal. "Nature decides whether or not an object is a planet."

The big issue for some is in the details, for example, Charon, formerly Pluto's moon, is round and defined as a planet because the two objects orbit a point in space (i.e., the two objects center of gravity) outside the surface of the larger object, Pluto -- so Pluto and Charon are actually a binary planet, both orbiting the Sun. Interestingly enough, if Plutos two other moons (Hydra and Nyx) were large enough to be spheroid (they just might be, depending on their composition, see more below), they would also be planets.

The asteroid Ceres regains its 19th century planetary status as the fifth planet because it meets the criteria, although the adjective "dwarf" is also suggested to use in reference to it. Pluto and Charon are also dwarfs, and also "Plutons", meaning a planet outside the orbit of Neptune. UB313 ("Xena"), bigger than Pluto, also qualifies, although discover Mike Brown of Caltech (homepage here) doesn't like the idea too much, particularly since worlds made of ice rather than rock wouldn't necessarily need to be much bigger than 400 km (being less dense) to become spherical, making possibly as many as 80 objects qualify.

"It's flattering to be considered discoverer of the 12th planet," Brown said in a telephone interview. He applauded the committee's efforts but said the overall proposal is "a complete mess." By his count, the definition means there are already 53 known planets in our solar system with countless more to be discovered."

Brown would also be the discover of several other planets as well, having found a number (five if I count correctly, but maybe as many as fifteen if any small icy KBO dwarf plutinos wind up qualifying) of the largest Kuiper Belt objects outside the orbit of Neptune. In addition, Ceres might not be the only asteroid that makes the "dwarf" planet cut. Currently there are a dozen other planets being considered in the asteroid belt and Kuiper Belt.

"And if astronomers determine that asteroids Pallas, Vesta, and Hygeia are also round, "they will also have to be considered planets," said Owen Gingerich, an historian and astronomer emeritus at Harvard who led the committee. The IAU proposal suggests (but does not require) that these be called dwarf planets. "

In addition to the three asteroids between Mars and Jupiter, you have the larger KBO objects Ixion, Varuna, Quaoar, Orcus, Sedna, and the officially unnamed objects 2002 AW197, 2002 TX 300, 2005 FY9, and 2003 EL61 being considered by the IAU as "candidate planets" as a part of the current proposal. Several other objects would appear to have been at least neglected so far, Brown has a list here of other possibles after some discussion at the top about the new definition and some new solar maps (new one is very busy, heh). Looks like the IAU was looking at object thought to be over 750 km, but missed 2002 UX25. Another four come in at just under that, from around 700-740 km.