The latest on the solar system: a proposal for 12 planets, which would reintroduce Ceres as a planet (ha! Take that, history!), albeit as a “dwarf planet,” reclassify Pluto’s moon Charon as a planet, and dub both Pluto and Charon as “Plutons,” to distinguish them from “dwarf planets,” I suppose, although apparently they will be both plutons and dwarf planets. Then 2003 UB313/Xena would also be made a planet, of the Pluton/dwarf species.
Seems little overcomplicated, particularly this Pluton/dwarf business, but it’s nice to see a consensus beginning to form around having Pluto and its ilk remain planets. Scott Westerfeld and I have (genially, to be sure) gone around as to whether labeling Pluto a different sort of planet is the first step toward a demotion or not; I think not because there’s 66 years of momentum behind the “Pluto as a planet” meme, and because Scott’s protestations notwithstanding, there’s not a thing wrong in noting that being small and icy and having eccentric orbits is a distinguishing characteristic of being a Pluto-series planet.
As I’ve said in the comment threads, what I think will eventually happen is that there will be nine “Historical Planets” that get named in popular astronomy books, with Pluto/Charon being considered one entry (possibly ten if popular imagination re-promotes Ceres), and then all the other planets get a hand-wave, as in: “Our solar system is comprised of nine historical planets, and many other smaller, icy planets discovered after Pluto.” Done and done. Among other things, this will allow people not to worry about screwing up the “naming the planets after Roman gods” thing.
Another interesting thing about this proposal is it seems to want to classify whether planet-like objects are planets or moons precisely as I did in the comment thread last night: By locating the center of gravity. If the center of gravity between two objects is inside the larger object, the smaller object is a satellite; if the center of gravity is outside of either object, both objects are planets in a double-planet system. Thus, our moon stays a moon, because the center of gravity for our earth-moon system is under our planet’s surface. But Pluto and Charon become a double planet. Works for me.
The vote on all this is eight days from now; I’m sure we’ll here more about it between now and then. Personally, I think it’s fairly neat this discussion is being picked up and carried over to a larger audience than these sort of things usually get — the “12-planet” proposal was the lead story on both the MSNBC and CNN Web sites this morning; apparently it’s too early for the “people killing the hell out of each other for no good reason” stories. Never fear. They will come. In the meantime, I wonder what the Vegas odds are for “Pluto stays a planet.” I’d bet.
Incidentally, the picture above, which shows the planets to scale (if not in their orbits), points out the real fact of the matter, which is that the solar system has four planets, and also a bunch of tiny orbiting rubble, some of which we just happen to live on. There’s perspective for you.