Why, then, did the I.A.U. demote Pluto? As David Spergel, the head of the astrophysics department at Princeton University, explained to me, once scientists discovered the Kuiper Belt, which includes several Pluto look-alikes, and once they discovered Eris, a dead ringer for Pluto, the organization became worried about a slippery slope. If Pluto was a planet, Eris would have to be, too, along with any number of Kuiper Belt objects. Things risked getting out of hand. Fifteen or twenty or fifty planets was too many—who would be able to remember them all? That last question may sound absurd, but in a debate held last year at the Harvard-Smithsonian Center for Astrophysics, Gareth Williams, the astronomer representing the I.A.U.’s position, couldn’t come up with a better argument. “You’d need a mnemonic to remember the mnemonic,” he said. “We really want to keep the number of planets low.” He lost the debate on the merits, but the demotion had already been won.