The votes are in, and it's a sad day indeed for Pluto.
At a stroke, poor Pluto goes from being the smallest planet to not even being the biggest dwarf planet.
But on balance, this new classification is a good thing. And even though astronomers have said that this definition only applies to our own solar system, I think it might be useful for classifying extrasolar planets as well. That is, once our technology is good enough to resolve such distant objects with sufficient accuracy.
The new definition categorizes a planet not just by size and shape, although it has to be big enough to pull itself into a spherical shape. It also has to be big enough to clearly dominate its region of space. This, in the end, is what kicked Pluto out.
Pluto does not dominate its region of space. Pluto is locked in orbital resonance with Neptune, which is clearly the dominant partner in that area of the Solar System. Several other objects in the Kuiper Belt are also in a 3:2 resonance with Neptune. Be that as it may, it clearly establishes Pluto as ruled rather than as ruler.
So, now we know. The arguments are over. But, I've got a sneaking suspicion that even now, after the votes have been counted, that the status of Pluto will still be good for an instant riot at any astronomers' gathering. Pluto's supporters will have to lick their wounds for a while, but they'll be back, and spoiling for a fight. Just you wait and see.
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This classification is NOT a good thing. It was done by four percent of the IAU, most of whom are not planetary scientists. It's sloppy in that it states that a dwarf planet is not a planet at all. An object does not need to dominate its orbit to be considered a planet. This was never a consideration before and was only raised by dynamicists who want to define objects solely by where they are rather than what they are. Objects like Pluto, Eris, and even Ceres are fundamentally different than asteroids and minor planets in that they have enough gravity to have pulled themselves into a round shape, a state known as hydrostatic equilibrium. These objects have geological processes, unlike asteroids. Most Kuiper Belt Objects are tiny rocks and should not be lumped together with the larger, round ones. The better thing to do is establish a new subcategory of planets, the ice dwarfs, for objects like Pluto, Eris, and Ceres. They are still planets, just of a different type. The current classification completely overlooks the issue of what objects are made of, which is why it was immediately rejected by over 300 professional astronomers in a petition led by Dr. Alan Stern, Principal Investigator of New Horizons.
Astronomers are not the only ones actively seeking to reverse this decision. There is also a strong popular movement to get it reversed, possibly as soon as the next IAU General Assembly. I'm a proud Pluto supporter, as can be seen from my blog at http://laurele.livejournal.com
It's not so much that we're back as that we never gave up the fight. Pluto is a planet, now and forever.
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