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Tyson, ever the iconoclast, thinks the word planet should be retired entirely, not just stripped from Pluto. "You tell me something's a planet," he says, "then I have to ask you 20 more questions to figure out what it actually is." As an educator as well as a scientist, though, he is thrilled that the question of planethood has been opened for freewheeling public discussion. "The point," says Tyson, who is working on a book about the Pluto debacle, "is that the solar system is a lot more interesting than just a list of nine planets." And thanks to Michael Brown and his associates, that fact is impossible to avoid.