Anything Smaller Than A Gas Giant Is A Trivial Space Rock, Anyway.

So, Loyal Reader Magnus dropped a comment on the last post asking me what I thought of this article about the International Astronomical Union's proposed redefinition of the word "planet." I suspect I'm the last person with a blog and an interest in space science to talk about this issue, but I honestly can't seem to get especially worked up about it. It's a question of terminology rather than one of science... Ceres isn't going to change if we start calling it a planet instead of an asteroid, and Charon isn't going to suffer an identity crisis if it's suddenly reclassified as half of a double planet instead of as a moon. And, hey, the proposed definitions seem reasonable to me, in that they're trying to be as non-arbitrary as possible.

I get the impression that a lot of people are sort of bothered by the whole thing, or are viscerally resistant to the idea of reclassifying. And I do understand that. As kids, one of the first things we learn about the universe beyond Earth is that we're part of a solar system with nine planets orbiting the sun. It's learned so early and taught with such confidence that it feels like a fundamental fact, not like an artifact of language. And it seems to form part of our basic sense of identity as a planet, if there is such a thing. The plaques on the Voyager probes show a diagram of one large circle and nine smaller circles, with the probe flying away from the third one to show any randomly encountered aliens where it came from. And it's easy to imagine ourselves in some Star Trek-ish future, chatting with an alien, going, "My solar system? Yeah, we've got nine planets..." It just seems like part of what defines this place. I remember as a kid reading occasional SF novels in which a tenth planet was discovered, and that always seemed like some kind of symbol of expanding human knowledge, of our growth as a species out into the universe. "Look, we found another planet out there! There's more to our solar system than we thought! We're expanding our horizons, and our solar system literally gets bigger as our knowledge grows! Ain't that something? Humans rock!"

Reality, of course, is always much messier. Hey, look, there are rocks out beyond Pluto! But they're dinky, and somehow the thought of letting them into the planetary club triggers less of a "Whoo, we have more planets!" response and more of a "Well, if those guys count, the whole concept of a 'planet' seems diminished somehow." The truth is, we like things tidy. We like for things to fit into neat categories and we like for the categories we grew up with not to change. I think most people would like to be able to name nine objects and feel like, if they know those nine things, they've got a handle on the solar system. It's much less simple and satisfying to view it as a huge swarm of bodies ranging from a dust speck to a gas giant, made of rock or gas or ice or various combinations thereof, orbiting the sun, orbiting other bodies, orbiting one another, here, there and everywhere, with no clear cutoff point where you can say, "This rock is a place, but that one is just a rock."

Like I said, I think scientifically the terminology doesn't really matter all that much, and part of me find the idea that there's controversy over it just kind of silly. But, y'know, if the debate gets ordinary people to start thinking about this stuff, to shake themselves out of the oversimplified "nine planets and some rocks and snowballs" model we all learned in elementary school and to understand the solar system a bit better as the gloriously messy thing it is, that seems like a good thing to me.

So there ya go. My two cents on planetary classification. Sorry you asked, Magnus?