How I Killed Pluto and Why It Had It Coming - Mike Brown [91]
According to the IAU proposal, though, the obscure fact that the center of mass of the Pluto-Charon system sat a bit outside Pluto rather than a bit inside it made all the difference. It suddenly turned Charon into a full-fledged planet, and Pluto-Charon into the solar system’s only double planet. Pluto lovers everywhere would be thrilled. Pluto’s status was about to change from imperiled to wildly distinctive. It would suddenly be the only place in the solar system you could go and get two planets for the price of one.
Except, of course, that the proposed definition was crazy. The members of committee first argued that only the object itself, and nothing else nearby, should be considered in determining whether the object was or was not a planet. Then they changed their minds and argued that satellites, though round, were not planets, because they were in orbit around larger round things instead of the sun. And then they changed their minds again and said that Charon, though small in comparison to other satellites in the solar system, was a planet because the common center of mass of the Pluto-Charon system was outside Pluto rather than inside it, so that, technically, Charon orbits an empty spot in space rather than Pluto. Because it doesn’t orbit a planet, it was therefore not—by this argument—a satellite.
So here is how you tell, in the committee’s opinion, that something in the vicinity of the sun is a planet. Look at it and see if it is round. If it is, then it might be a planet. Next, check to see if it orbits around something else instead of the sun. If it does, then it’s probably just a moon and not a planet. But before you know for sure, calculate the center of mass (if you even know the masses of the bodies in question, which you usually don’t) and see if it is inside or outside the larger body. Then you know. It’s all quite simple.
While the inclusion of Charon was the most jarring aspect of the proposal, there was one other oddity that I couldn’t make sense of. The committee said that all round things were planets (except for moons, which weren’t, except for Charon, which was). I had estimated that about two hundred objects in the solar system would fit that criterion, but the IAU had done its own estimate and come up with its own number: twelve.
Why would Charon and the asteroid Ceres be added, but not the dozen known Kuiper belt objects that were larger than Ceres? And the hundreds that were smaller but almost certainly round? It was as if the International Arboreal Union were to tell you that all things with trunks and bark and branches and leaves were to be called trees, but then it told you that the only trees were oak trees, maple trees, and elm trees. You would be right to ask: How can you make a very precise definition of tree and then claim that things that very precisely fit your definition are not, in fact, trees?
Why would the International Astronomical Union do such a thing? I have a theory that I strongly believe to be true, but which is strongly denied by everyone I’ve talked to who might have more intimate knowledge of how the decisions were made. My theory is that the IAU decided that keeping Pluto as a planet and adding three new planets—Xena, Charon, and Ceres—would seem like a minor change to the order of things. It knew that after the newspapers declared that the solar system now had twelve planets and it proudly exclaimed that its new definition was the first true scientific definition, the pro-Pluto crowds would be satisfied and no one would be terribly startled. Three new planets? Yeah, that happens every century or so. No need to get alarmed. Who could complain? It wouldn’t elicit anything like the reaction people would have to the headline “Solar System to Have 200 Planets!” Given the choice between scientific rigor that might cause protests and a scientific whitewash to conceal reality, the IAU