Whether Pluto is a planet or not. Passionate discussions on the matter are here, here, here, and a real good summary of the entire huggamugga here.
Jeez, we're such geeks.
Here's the deal. The International Astronomical Union (IAU) is trying to define what a planet is. That should be simple, right? A planet is an object that goes around a sun. Unless its another star. Or an asteroid. Or a comet. Or a moon that's going around a planet that's going around the sun.
Hmm. OK, it's not so simple. And to my mind the question is one of those mug's games, one of those situations which divides the knowledgeable and holds them up to ridicule by the ignorant. I mean - scientists! - they can't even figure out what a planet is!
But they walked into this mess, and now they're going to have to use their big juicy scientist minds to get themselves out.
The problem is Pluto. Discovered in 1930, its always been the oddball world. An inclined eliptic (about 11 degrees off the sun's equator). Smaller than Mercury. Orbit that takes it within Neptune's. Very different than the gas giants immediately sunward. Its always been the asterisk in our cosmology, the strike-crippled 1994 baseball season of astronomy. Various folk have suggested it was a runaway moon, but it was let into the solar family, the runt of the litter, and it drew a pleasing coda to the entire nine bodies.
Except it didn't. We have been dealing with an increasing number of 10th planets over the years, usually heralded with a Page 4 announcement and then forgotten about until someone discovers a NEW 10th planet. Xena (UB313, also called Lila), Orcus, Sedna, Quaoar, Santa (EL61), and Easterbunny (FY9). And they're allowed to count, since, after all, we let Pluto in. Why not let them in?
So what is a planet anyway?
The scientists of the IAU get to wrestle with this while the rest of us make cheap jokes at their expense. After a lot of deliberation, here's what they proposed:
A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet..
Ok, that sounds good, but it lets in not only Pluto, but Xena/Lila (which is larger than Pluto after all), and the asteroid Ceres (which is a roundish spheroid) and even Charon, Pluto's major moon, which fails the satellite test (the balancing point between the two gravitational wells is between the planets, which makes it a double-planet, as opposed to the Earth-Moon combo, where the balancing point is still beneath the Earth's surface).
So suddenly we have the door swung wide and have 12 planets, two of which orbit each other, one of which shares a region of space with a lot of other, smaller, objects, and we have more knocking on the door every year. If Ceres is in, why not Vesta? Or Pallas? Or even Chiron?
But by the same token kicking Pluto out of the brotherhood doesn't seem right - its like disowning a baby brother because it turns out the adoption papers were botched. Similarly, rejiggering the rules feels weird in that it will sudden "make" Ceres a planet, and grants the late-comer Charon equal billing with its longer-known sibling.
And I'll admit I don't have a good handle on all this - I was thinking of confining the planetary definition to a roughly circular orbit, but that makes Quaoar more of a planet than Pluto/Charon (but knocks out the almost cometary Sedna). And the same for orbital inclination - Ceres is a bit more in the eliptical plane than Pluto/Charon is.
But all this point out the danger of science - it wants to be precise and accurate in a world that is sloppy and unapoligetic. The end result is that the IAU will provide a definition, take some hits and most of the rest of us will deal with nine planets, just like most of us still think of atoms a little tiny solar systems.
And of course, we're just going to discover another jovian gas giant out there in the Kuiper Belt, and suddenly everything is going get knocked around again.
(Apologies in all this to Shelly in Seattle, the Monkey King, Ironymaiden, Oni Anne and Sigtrent, who had to hear some of this already over brunch)
[And the ghost of Dave Bowman passed over this blog and stated "My god, its full of links"]