'Tenth Planet' Bigger Than Pluto

It's official — the so-called "10th planet" discovered last year is bigger than Pluto.

However, that may not mean much, since astronomers can't agree on exactly what a planet is, or even if Pluto itself qualifies as one.

The discoverers of the "planet," a Kuiper Belt object semi-officially known as 2003 UB313, gave only a rough estimate of its size, based on its brightness, when they reported their findings in July 2005.

That team — Michael Brown and Chad Trujillo of the California Institute of Technology and David Rabinowitz of Yale University — found the object, and its moon, by analyzing photographs taken by a telescope at the Mount Palomar Observatory in California in 2003.

A German group of researchers has now used a different form of analysis to more accurately gauge 2003 UB313's diameter.

By measuring how much heat the planetoid radiates, the scientists led by Frank Bertoldi of the University of Bonn estimated that 2003 UB313 is about 1,864 miles across.

That makes it larger than Pluto, which has a diameter of about 1,429 miles.

"It is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status," Bertoldi said in a statement.

Details were published in Thursday's issue of the journal Nature.

Brown said the Germans' measurement seemed plausible and said his team was using the Hubble Space Telescope to directly figure out its size.

He and his associates had originally reported that 2003 UB313 was thought to be larger than Pluto and estimated that it was most likely between 1,398 miles and 2,175 miles in diameter.

Among themselves, Brown, Trujillo and Rabinowitz have referred to the main body as "Xena," after the sword-wielding syndicated TV warrior princess. The moon naturally has been called "Gabrielle," after the TV Xena's trusty sidekick.

Neither name is likely to stick, however. By convention, all new planets and Kuiper Belt objects are named after gods of various mythologies, and "Xena" and "Gabrielle" don't qualify.

The previous contender for 10th planet, Quaoar, discovered by Brown, Trujillo and Rabinowitz in 2002, was named after the creator god of an American Indian tribe that lived in Southern California. It is about as far from the sun as Pluto is but has only one-third the mass.

Astronomers have been debating for several years over what a planet is and whether Pluto should keep its status.

A new model of the solar system at the American Museum of Natural History in New York leaves out Pluto, which prompted a letter-writing campaign from schoolchildren to reinstate the ninth planet when the permanent exhibit is unveiled.

The difficulty is that there is no official definition of a planet, and some argue that setting standards such as size limits would open the door too wide.

When the largest asteroids, Ceres, Vesta and Pallas, were discovered in the early 19th century, they were counted as planets until it became clear that they were part of a much larger belt of large rocks and planetoids ringing the Sun between Mars' and Jupiter's orbits.

Likewise, Pluto and Quaoar are part of the Kuiper Belt, an even larger ring of small objects orbiting the Sun beyond Neptune.

The Associated Press contributed to this report.