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Tyson's heretical planetarium exhibit was based on the theory that there must be lots of things out there the size of Pluto. Brown and Trujillo found some of them, and for the past year and a half, the pressure has been on to decide once and for all which are planets and which are not. 2003 UB313 just upped the ante. But it is like trying to define continent, says Brown. "Some geographers call Australia a continent," he says, "and some call it a very big island. There is no scientific definition." It is human nature to put things into categories, but nature rarely cooperates. What, precisely, is the dividing line between a hill and a mountain? A rock and a boulder? A stream and a river?
Most people don't worry much about such distinctions. With planets, however, it's different--as Tyson discovered. How do you resolve the problem he created? One idea would be to arbitrarily set the lower limit for a planet at about 2,000 km (1,250 miles) in diameter, which would let Pluto remain a planet and make 2003 UB313 one as well, but keep the rest of the riffraff out. "Pluto," says Alan Boss, an astronomer at the Carnegie Institution of Washington and a member of the IAU working group, "has historically been considered a planet, and so any definition we adopt really must include it." Another proposal would drop the limit to 1,000 km, letting Quaoar and Sedna into the club as well.
Yet another idea, favored by Alan Stern of the Southwest Research Institute in Boulder, Colo., would open the door even wider. By his definition, any object massive enough for gravity to squeeze into a spherical shape is a planet--unless the object orbits a bigger planet, of course. Otherwise, dozens of moons would have to be reclassified as planets. "Defining planets by size is purely arbitrary," agrees Marsden, who likes Stern's idea. "The Pluto-crats want to cut things off there, but it's absurd to say that an object 2,000 km across is a planet and one 1,999 km across isn't."
The roundness rule would add lots of planets to the solar system in one fell swoop: not just Sedna, Quaoar and 2003 UB313 but also two more icy worlds spotted by Brown and Trujillo--2004 DW, a little bigger than Quaoar, and 2003 EL61, probably about seven-tenths the size of Pluto. The latter made headlines when it was formally announced to the world by Spanish astronomers who, according to Brown, knew where to look because they had used the Internet to tap into his telescope logs (the Spaniards deny the charge). At least five or six asteroids would also qualify, says Marsden. There would probably be two dozen newly designated planets in all.
There's no telling when the IAU might make a decision. It could be as early as the end of this month. But it can't wait forever; Brown and Trujillo have even more discoveries waiting in the pipeline (they've put their logs behind a firewall to keep prying competitors away) and they're not done yet. Just about all the new worlds have been found by looking even farther outside the plane of the solar system than Pluto's orbit. "Nobody really expected to see anything way up there," says Brown. "But based on what we've found so far, we expect to find at least two or three more of these."