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Moons Do Not A Planet Make

OK, I am happy to give back Pluto, since it has 4 moons. But only if I get the pathetically moonless Mercury and Venus. — Mike Brown, AKA @plutokiller

Because I can’t leave well enough alone, I have a few more quick thoughts about the new moon discovered orbiting Pluto:

  • A lot of people are saying that the fourth moon orbiting Pluto should tip the balance on its planethood. To elaborate on a point I made in the original post: by that argument, none of the terrestrial planets should be considered planets: Mercury and Venus have no moons, Earth has only one, and Mars has two little bitty tiny moons (Phobos and Deimos). Any planetary scientists reading this should jump in if I’m wrong (or to tell me if I’m right!), but it seems that the number of satellites a body has is more a function of its location than anything.
  • To quote myself on Google+ (and yes, you may add me if you so desire), “My students often conclude that there shouldn’t be a unitary definition of planet, but a two- or three-part definition: terrestrial planet, Jovian planet, and (optionally) ice planet. They often decide (reasonably, in my opinion!) that Jupiter is so dissimilar to Earth that they aren’t the same kind of beast at all.” I may come back to this specifically in a future post, but I really think I need to read Mike Brown’s book soon — he’s a smarter dude than me, and has thought about this issue more than I have.
  • And again, I have no personal stake in whether Pluto is considered a planet or not. The main thing I care about is not falling into the attitude of “I learned 9 planets in school, so that’s good enough for me!” That attitude is no better (or different in effect) than the Creationist “God said it, I believe it, that settles it” argument.
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