Fifth Planet

Under a proposal before the IAU, a formal definition of planet is to be voted on. Very briefly the proposal sets forth criteria which touch on orbital and dimensional characteristics of non stars. The motivation for considering the proposal has been the discovery of Pluto like objects outside of the orbit of Neptune. While the deep space Kuiper Belt bodies brought on the debate, the solution could bring the Asteroid Ceres back into the fold of the planets in our Solar System.

Ceres was considered to be a planet after its discovery for most of the 19th Century. At the time it seemed to fit a hole in "Bode's Law" on orbital spacing of the planets. The discovery of other objects in similar orbits and the inability to resolve a disc resulted in Ceres being reclassifed as a "little star" (i.e. asteroid). Bode's Law is no longer seen as a physical law, but more of a quirk. Similarly, Pluto, isn't the "Planet X" that astronomers were looking for in the 1920's.

Today we know that Ceres has something like a third of the total estimated mass of the main belt. Hubble Space Telescope images of Ceres show it to be round, and Ceres is thought to have a differentiated interior and possibly an atmosphere. All of this makes it far more of a standout among "asteroids" than Pluto/Charon appear to be among Kuiper Belt bodies. (note that I'm not saying Pluto doesn't also have some of these features.) Ceres' orbit is whole lot more regular than the Kuiper belt bodies. And though I would have prefered to wait until the Dawn probe confirmed (or killed) some of the speculation about Ceres, I find it no harder to call Ceres a "planet" than Pluto/Charon or 2003UB313.

The IAU proposal at least provides objective criteria for designation as a Planet which don't appear, at first blush, to be completely arbitrary. I would agree that the draft proposal was influenced by a desire to "save" Pluto's designation, but having done so, it was almost impossible to avoid sweeping in Ceres without looking like the desired result was controlling the selection of criteria. If orbital criteria (e.g. dominant body among those in similar orbits, etc) had been added to the proposal it would have easier to write off Pluto than Ceres.