How I Killed Pluto and Why It Had It Coming - Mike Brown [101]
Most people, though, don’t have specialized interests in the solar system. The only classification scheme they will ever know is the word planet. They will know what a planet is and how many planets there are and what their names are. Their entire mental picture of what the solar system is, of how our local bit of the universe is put together, will be carried in the understanding of that simple word. The definition of the word planet, then, had better carry with it the most profound description of the solar system possible in a single word.
If you think of the solar system as a place consisting of eight planets—or, better, four terrestrial planets and four giant planets—and then a swarm of asteroids and a swarm of Kuiper belt objects, you have a profound description of the local universe around us. Understanding how such a solar system came to be is one of the major tasks of a wide range of modern astronomers. If, on the other hand, you think of the solar system as a place with large things that are round and smaller things that are not quite round, you have a relatively trivial description of the universe around us. There is nothing important to study here: We’ve known for hundreds of years that gravity pulls big things in space into the shape of a sphere.
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Sometimes you don’t even have to go through such extensive arguments. If you catch a person early enough, before the idea that Pluto deserves to be a planet has sunk in, you can teach things correctly from the start. Take Lilah, for example. Everywhere I went in the months following the IAU decision, people wanted to know if I thought Pluto had been treated fairly. Did I think Pluto was a planet? After a few weeks, I taught Lilah to answer for me.
“Lilah, is Pluto a planet?” I would ask, beginning our choreographed banter.
She would frown and shake her head.
“No no no no no no no.”
As she got older the banter continued: “So what is Pluto, Lilah?”
“He’s not a real dog. He’s a dwarf dog.”
My friends would laugh, and then invariably go out and buy Lilah Pluto toys. She has stuffed dogs, of course, but also a collection of nine-planet memorabilia. Early on she learned to figure out which one of the nine little circles on whatever picture she had was Pluto and then promptly declare, “Pluto is a dwarf dog.” The continued laughs from that line were more reinforcement than I could possibly have given.
Another friend was worried how Lilah would react when she got older and discovered that I was a planet killer. “What will Lilah think,” the friend said, “when she learns that Pluto is not a planet and that you are to blame?”
“I know what’s going to happen,” I replied. “In second grade or third grade, when she learns about planets she’ll come home and say, ‘Daddy, today we learned about the eight planets,’ and I’ll say, ‘Lilah, did you know that when you were born we thought there were nine or even ten planets?’ She’ll look at me, shake her head, and say, ‘You know, adults are so stupid.’ ”
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Now that Xena, too, was officially called a dwarf planet, it finally got a real name. The possibilities were wide open, but Chad, David, and I had decided that because—at least in our minds—Xena had been the tenth planet in good standing for an entire year, we wanted to give it a Greek or Roman name, like all of the other planets have. The problem was that there were very few left to go around. Back in the 1800s, when asteroids were first being discovered, they were, of course, called planets. And people wanted them to have Greek or Roman names, like the other planets. So they used up almost all of the major gods and goddesses and most of the minor ones, too. Every time we found a name we thought might be nice, we