Astronomers in the US reckon they've identified a tenth planet orbiting our sun. The body is around between 2,600km and 3,000km across, and inhabits a region three times as far from the sun as does Pluto, averaging a distance of nine billion miles from our star. A single orbit takes the world 560 earth-years.

Officially designated 2003 UB313, and now more affectionately named Xena, it was originally discovered in January by Caltech's Michael Brown, Chad Trujillo from the Gemini Observatory in Hawaii, and David Rabinowitz from Yale University. The researchers say confirming details about the world has taken some time.

The classification of the body as a planet is likely to be extremely contentious because although the body is bigger than Pluto, it is a Kuiper belt object (KBO).

The Kuiper belt is an icy disk of debris orbiting the sun, beyond Neptune, and is the original home of many comets. The objects are most likely very primitive: detritus from the early accretion phases of the solar system.

Many scientists would prefer that all KBOs were designated as asteroids, or for the very large ones like Pluto and Xena, as minor planets. The discovery is likely to re-ignite the debate over exactly what a planet is. Currently, astronomers don't really have a rock-solid definition that everyone agrees on.

Indeed, some argue that Pluto itself is not a true planet, and that it would be better classified as a minor planet as well.

Planet or otherwise, the discovery of such a large object so far from the sun is not an everyday occurrence. Chad Trujillo told the BBC: "I feel extremely lucky to be part of a discovery as exciting as this. It's not every day that you find something Pluto-sized or larger!"

Caltech's Michael Brown was expecting to find something like Xena out there. He told The Observer that he had narrowly lost a bet with a fellow astronomer that he would discover a KBO larger than Pluto by the end of 2004. Eight days later, along came Xena.

"I lost the bet by eight days," Brown told the paper. "But she graciously decided she would let that window slide and I would win the bet. That means I get to drink 10 bottles of good champagne. And I think I will." ®

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