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The Year in Science: Astronomy
[Discover, January 2006]
By RICHARD MORGAN
For schoolchildren trying to memorize the names and order of the planets, life just got a bit harder. In July astronomer Mike Brown of Caltech announced the discovery of UB313, a body larger than Pluto, orbiting at about three times Pluto's distance from the sun. By Brown's reckoning, the object qualifies as a 10th planet. Some of his colleagues counter that UB313 is more like an outsize asteroid—and many of them want to demote Pluto while they're at it. Either way, the solar system is looking both larger and stranger than we thought.
The confusion began in 1992 with the discovery of the Kuiper belt, a long-theorized second zone of asteroids on the edge of the solar system. Many of the objects there seemed similar to Pluto but smaller. Then Brown spotted UB313, which is about 1,700 miles wide, or about one-quarter bigger than Pluto. If Pluto is a planet, then UB313—which Brown nicknamed Xena—logically should be as well.
By most recent estimates, however, there are probably another two or three objects in the Kuiper belt as large or larger than Pluto, which will further complicate the picture. "Look, I don't care. It's semantics," says Hal Levison, a planetary scientist at Southwest Research Institute's space science department in Colorado. "But if you come up with a strict scientific definition, you either end up with eight planets—Pluto would not make it—or a lot, like more than 20."
Brown favors the eight-planet solution, but he acknowledges that his is not necessarily the most popular view. "The way to make astronomers look stupidest is to declare that Pluto, this thing that's been a planet for 75 years, isn't one," he says. "The bright side is that this is an amazing opportunity to let people see science as a living thing that makes discoveries and changes."
Even Xena's name remains in limbo while scientists scramble for a vocabulary to match their findings. Brown's provisional moniker comes from the canceled television show Xena: Warrior Princess, but a formal name has yet to be certified by the International Astronomical Union because of the putative planet's disputed status (Xena also does not fit the union's naming convention). The added discovery in October that Xena has a moon probably ups its chances of gaining recognition as a planet. "The public tends to think that any self-respecting planet should have a moon. It's just cooler," Brown says. Meanwhile, he has nicknamed the moon Gabrielle—in honor, naturally, of Xena's TV sidekick.
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