Several of my co-bloggers have mentioned this already. The International Astronomical Union (IAU) has decided that Pluto — while still technically a planet in spite of its unique composition (ice) orbital plane (inclined 17 degrees) and orbital eccentricity (about 0.25) — now merits its own planetary subcategory, which the IAU has, aptly enough, dubbed “plutons” (although I would have preferred “plutelets”). A planet, according to the IAU, 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, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet”; plutons, meanwhile, are planets with “highly inclined orbits with large eccentricities and orbital periods in excess of 200 years.”
As a result of this new definition, three new planets have magically been “found” in our solar system: Ceres (the largest of the asteroids, which orbit the sun between Mars and Jupiter); Charon (formerly considered a moon of Pluto); and UB313, a chunk of something-or-other even further out in the celestial boondocks than Pluto and dying for a more snappy handle.
Actually, now that we’re up to an even dozen, why not rename all of them? Let’s see…Optos, Ophthalmos, Oculomotos, Trochleus. Trigeminus, Abducus, Facius…nah, that won’t work. Okay, how about Peter, Andrew, James, the other James, Judas, the other Judas, Bartholomew…on second thought, let’s just go with what we have now.
Anyway, the inappropriately named Bad Astronomy Blog has the complete scoop.