How I Killed Pluto and Why It Had It Coming - Mike Brown [100]
As you start to stare at and wonder about the sun, you suddenly notice that—wait!—the sun is not alone! You see that there is something tiny right next to it. You’re excited beyond alien words. As your spaceship gets closer and you look even more carefully, you suddenly realize there are two tiny things next to the sun. No, three. No, four!
You have just found the things that we call Jupiter, Saturn, Uranus, and Neptune: the giant planets. From your perspective, still quite far from the solar system, they look tiny and so close to the sun as to be barely distinguishable. You don’t have a word to describe them, so you make one up in your alien language: Itgsan.
You keep looking for a fifth Itgsan out beyond that fourth one you found, because it seems logical that there should be more, but even as your spaceship gets closer and closer to the system, you don’t see anything out there. Trust me, I understand your disappointment.
Finally, as you get close and the four Itgsan get brighter and appear more distinguishable from the sun, you realize you were looking in the wrong place all along. There are other things next to the sun, but they are inside the first Itgsan, not outside. There are four of them, but they’re much smaller than the first four things you found. So you come up with a new word. You call them Itrrarestles. You don’t know it, but you’ve just found Mercury, Venus, Earth, and Mars.
For a very long time, as you keep getting closer, there is nothing new. Finally, when you’re almost on top of the solar system, you realize that between the small Itrrarestles and the large Itgsan there is a band of millions and millions of tiny things going around the sun. And looking even more carefully, you see that outside the large Itgsan there is another band with even more. You call them something that I can’t pronounce, but I call them the asteroid belt and the Kuiper belt.
Nowhere in that alien brain of yours would it be likely to occur to you to take one or two or even a few hundred of the things sitting in the Kuiper belt or in the asteroid belt and put them in the same category as the big things, the Itgsan and the Itrrarestles. Instead, you would quite rationally declare that the solar system was best classified by four major categories. And you would, I think, be correct.
The only thing wrong with our current classification of the solar system as a collection of eight planets and then a swarm of asteroids and a swarm of Kuiper belt objects is that it ignores the fundamental distinction between the terrestrial planets—Mercury, Venus, Earth, Mars—and the giant planets—Jupiter, Saturn, Uranus, Neptune. In the class on the formation of planetary systems that I teach at Caltech, I try to convince my students that, really, there are only four planets and that Mercury, Venus, Earth, and Mars shouldn’t count. But even students who worry about their grades aren’t willing to go that far. So, even though the aliens call them Itgsan and Itrrarestles, we’ll lump them together and just call them all Tsapeln.
You can classify anything at all in many different possible ways. If you are studying birds, you might split them into land birds and seabirds; carnivorous birds and seedeaters; red birds, yellow birds, black birds, and brown birds. All of these distinctions can be important to you, depending on what it is you are studying about birds. If you are studying their mating habits, you might classify them in categories of monogamous and polygamous. If seasonal migration is your thing, you could classify them by those that stay put and those that fly south for the winter.
Things in the solar system can equally well be categorized in many different ways. Things with atmospheres. Things with moons. Things with life. Things with liquids. Things that are big. Things that are small. Things that are