August 16, 2006

Twelve Planets?

While you were sleeping last night the solar system got an upgrade. You went to bed with visions of nine planets dancing in your head, the nine that have been taught to us in school for our entire lives. But as of today that list of nine has been thrown out the window, replaced with the new list of twelve. And no, there weren’t three new planets created out of swirling gas last night, it’s just that the rules for what can be called a planet have changed, and under the new rules we’ve let three new applicants into the club.

So what changed, and how did we end up with twelve planets? It’s something that’s been in the works over at the International Astronomical Union for a while now. They’ve been meeting behind closed doors and trying to figure out what to do about the fact that a KBO (Kuiper Belt Object) was found a few years ago that’s bigger than Pluto. Good old 2003 UB313, which can only be seen with a powerful telescope, is about 2,400 kilometers across. Just a smidge bigger than Pluto’s 2,300. So it was clear that new rules had to be pinned down, so that either both of them were planets or neither of them were. I wrote about all of this a few months ago.

The IAU has been working on this for months, and last night they announced their decisions. They’ve come up with a new category of planets, called “plutons”. A pluton is something that is out past Neptune, with a wild orbit, but is still round and planet-like. Pluto’s been added to this new category, as has 2003 UB313. And, in a big surprise, so has Charon, which is actually a moon of Pluto. Through some very creative accounting, they’ve decided that Charon really isn’t a moon, it’s actually a partner of Pluto, and the two of them make up a “double planet” system. The two are really orbiting each other, they say, instead of acting like a normal planet-and-moon system. That’s enough to give Charon a huge promotion in planetary status.

Is your head swimming yet? Well, grab hold of something because we’re not done. You’ll notice that adding Charon and 2003 UB313 to the list only brings us to a total of 11 planets. Where’s #12? That would be Ceres, which ever since the 1850s has been called an asteroid. It’s in the asteroid belt, between Mars and Jupiter, and it’s also so tiny that you need a telescope to see it. But it’s round, just like the other planets, and it fits the new definitions, so it got squeezed into the list. In fact, they invented a whole new category just for it; no longer an asteroid, now it’s considered a “dwarf planet”.

So there we have the new solar system, with twelve planets in all. And a whole host of new rules that pretty much guarantees that the number will go up. There surely are countless more “plutons” out there that we haven’t found yet, and each one of them will be added to the list as they’re discovered.

For much more (and far better) coverage, check out Phil Plait’s Bad Astronomy website. He’s got the whole rundown, as well as a few educated opinions on these new developments.

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