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.