The discovery of a distant object larger than Pluto orbiting the sun seems secure enough. How to pigeonhole it, though, is completely up in the air. Is it the tenth planet, the first one discovered since Pluto in 1930? Or is it, with Pluto, just another Kuiper Belt Object (KBO), one of the thousands of icy chunks of debris left from the solar system's formation?
Three astronomers--Michael Brown of the California Institute of Technology in Pasadena, Chadwick Trujillo of the Gemini Observatory in Hilo, Hawaii, and David Rabinowitz of Yale University, in New Haven, Connecticut--first photographed the cosmic bone of contention almost 2 years ago. This January, they noticed that the object, temporarily designated 2003 UB313, was moving against background stars and was thus orbiting the sun. They calculate that it is at the most distant point of its orbit--97 times as far from the sun as Earth is on average.
The new object is so bright that it must be larger than 2390-kilometer-wide Pluto, the group reports. But because the orbiting infrared Spitzer Space Telescope cannot detect it, the object must be smaller than 3200 kilometers.
Brown and colleagues are calling 2003 UB313 "the tenth planet." Some astronomers agree. "If it's larger than Pluto," says David Tholen of the University of Hawaii, Manoa, "I'd call it the tenth planet, because Pluto is the ninth planet by historical precedent."
But theoretical astrophysicist Alan Boss of the Carnegie Institution of Washington's Department of Terrestrial Magnetism thinks that would be a bad move. When the first asteroid, 946-kilometer Ceres, was discovered in 1801, he notes, astronomers called it a planet too. But they demoted it to minor-planet status after other asteroids started showing up between Mars and Jupiter. The new object, Pluto, and several slightly smaller KBOs discovered recently "are all part of one population of objects," Boss argues--no one of which has enough mass and gravity to dominate its region of space the way the first eight planets do.
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