Back in October I had the chance to give a talk in conjunction with Dr. Bruce Greenway, a fellow Michigander and an actual professional astronomer. He made the most convincing case I’ve seen for Pluto’s reclassification — demonstrating beyond the shadow of a doubt that Pluto has far more in common with objects like Eris, Ceres, and Pallas than bodies like Mercury, Mars, Earth, or other canonical planets. The difference in mass is crucial — and the degree to which this massiveness has allowed the principal bodies in question to clear the neighborhood around their orbits. If you’re wondering whether Pluto’s more like Eris or Mercury, this graph ought to make the case plain:
I’m cribbing this slide from Bruce’s PowerPoint. It points out exactly how drastically the difference in mass affects the amount of other “crap” in the neighborhood. And seems to suggest that — all legacy considerations aside — Pluto ought to be categorized with its fellow dwarves.
We ought also to be struck by how extremely unlikely it was that Clyde would have seen such a tiny little thing.
Finding Pluto 
December 13, 2010 at 5:13 pm
Alan Stern and Hal Levison did a similar study in 2000, dividing the group in the top half of your graph as “uber planets” and most of those in the lower half of your graph as “unter planets” or dwarf planets. However, they never intended for the “unter planets” to be classed as not planets at all. The idea is any object that orbits the Sun and has enough mass to be in hydrostatic equilibrium is a planet. Dwarf planets are simply a third class of planets encompassing objects large enough and massive enough to be in hydrostatic equilibrium but not massive enough to gravitationally dominate their orbits.
Pallas and Vesta do not really belong in either class. They are of an intermediate stage between asteroids and dwarf planets, as they appear to be nearly round but to have both lost a portion of themselves through impact with an asteroid. Since they are believed closer to planets than asteroids in that they are geologically more complex, some have proposed an intermediate category for them, suggesting terms like “protoplanets” or “sub-dwarf planets.”