The IAU has voted on a series of resolutions on what a planet is and what a planet isn't, and the verdict is...
Pluto is not a planet.
At least, not a major one.
This is a big turnaround from the initial resolution, which would have given our solar system at least 12 planets, and potentially many, many more. Here is the first resolution that passed:
RESOLUTION 5A The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
Ignoring for the moment, once again, that it's silly to try to scientifically define a class of objects that are really only defined culturally, these definitions are still unsatisfying to me. A planet-sized object between stars is not a planet? How round is round? How do you define its "neighborhood"? These are still the same objections I made before in my earlier post about this.
But I suppose what people want to know is how Pluto fits in this. Pluto is round, and orbits the Sun, but has not cleared out its local neighborhood. Smaller objects that orbit the Sun in nearly the same orbit will get absorbed by or ejected by the larger object. As planets form, their gravity either pulls in smaller bits of junk, causing them to impact, making the planet grow, or it slingshots the smaller object away, putting it in a very different orbit. That's why big objects in the solar system tend not to have anything else near them (except moons).
Pluto fails this. As I understand it (the news is still sketchy from the IAU meeting) there are other objects in similar orbits as Pluto, and therefore Pluto has not cleared out its neighborhood. I'm not sure if Charon, Pluto's moon, is included in that list of uncleared objects. Now, this is a little confusing: lots of planets have moons, so just having a moon doesn't mean a planet has not cleared its area (since the moon is bound gravitationally by the planet). But Charon orbits Pluto far enough out that the center-of-mass of the system is outside Pluto's surface (again, see see my earlier post about this). Ironically, with the original resolution, this made both Pluto and Charon a planet. Now, under the new rules, this may mean neither is.
So: according to the new rules, passed by the IAU, Pluto is no longer a planet. I guess Neil Tyson will have to go on Colbert again.
The IAU made this pretty official with another resolution:
RESOLUTION 6A The IAU further resolves:
Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
This sits better with me, actually, than calling Pluto a planet, but a lot of people aren't gonna like it.
Incidentally, there were two resolutions voted down:
Insert the word "classical" before the word "planet" in Resolution 5A, Section (1)
so that if it had passed we'd call the 8 major planets "classical". The other resolution would have been added to 6A about the dwarf planets:
This category is to be called "plutonian objects."
Since this last bit was voted down (narrowly, 187 to 183!), the IAU will decide what to call this class of objects at the next meeting, in Rio in 2009. Rio, hmmmm... maybe I'd better go to that one.
Let me once again reiterate that trying to define what a planet is is very, very silly. The very fact that all this is so bizarrely confusing is good evidence of this.
Want another reason this is silly? If the reason Pluto isn't a planet is because of Charon, then we're in trouble: as I pointed out in my other post, in a billion years or so the Moon will be far enough away that the Earth-Moon center-of-mass will be outside the Earth. So at that time, if I understand this correctly (and I may not), Earth will no longer be a planet. I need to find out more about all this, but as I said, details about why exactly Pluto isn't a planet anymore are still a little sketchy. I'll post more when I find out.
And here's another point. Pluto crosses Neptune's orbit. Due to the delicate dance of gravity between the two, they never actually get near each; Pluto is always on the opposite side of the Sun from Neptune when it crosses the bigger planet's orbit. So, if Pluto's orbit actually overlaps Neptune's, doesn't that mean Neptune hasn't cleared out its neighborhood? I think you might argue that. So why don't we have 7 planets?
I'm really torn over this. Scientifically, this whole debate is a tempest in a teapot. It's ridiculous, and serves no purpose. How is scientific knowledge furthered in any way by debating and resolving this?
On the other hand, it's gotten a lot of interest by the public, and it's been positive interest so far. People are talking about what it means to be a planet, and given the abysmal level of science education in the US, it's great that folks are actually talking about astronomy. Maybe it'll lead to some of them looking into it more, and that's a good thing.
And now, finally, just maybe, we can actually get back to studying these objects instead of arguing about what to call them. There's much to learn about them, real stuff, interesting stuff. The planets -- however many you may think there are -- are waiting. Let's get going.