Pluto

When I first heard about Pluto not being called a planet, I didn’t care much, but it did seem like an instance of scientists getting overly picky about technical terms for colloquial concepts. (This happens all the time in biology – like trying to distinguish between “true spiders” and other spiders. Like it matters – they’re all evil monsters [or as my brother would say, “pure darkness”].) But after listening to the professor in one of the introductory astronomy classes I took in college, I’m convinced. It’s not a planet. It shouldn’t be called a planet.

The beginning of the argument is that there are all sorts of icy rocks like Pluto orbiting the sun beyond Neptune (or crossing its path like Pluto does). They have weird orbits and sometimes become comets. Pluto is the biggest of these Kuiper belt objects, but that’s not really that special. What really convinced me, though, is the matter of Ceres. Have you ever heard of the planet Ceres? I didn’t think so. See, a while ago they discovered a big rock between Mars and Jupiter, roughly spherical and maybe even with some atmosphere. They named it Ceres and classified it as a planet. Then they discovered another rock. Then another one. Then another one. And pretty soon it became clear that Ceres didn’t count as a planet because it shared its orbit with all sorts of random little rocks. The fact that Ceres is bigger than the other rocks doesn’t make us want to call it a planet. It has hydrostatic equilibrium (an awesome way of saying that is has enough mass to smash itself into a sphere), but it hasn’t cleared its orbit of similarly-sized objects. So we don’t call it a planet.

The thing is, Pluto is just like Ceres, only a little bigger and a lot farther away. Yes, it has a moon, but so do some asteroids. So unless you want to lobby for calling asteroids “planets”, you pretty much have to accept that Kuiper belt objects don’t count either.