'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.