The controversial body mooted as a possible tenth planet orbiting our sun is around a third larger than Pluto, German astrophysicists claim. 2003 UB313 - aka Xena - was spotted last January by Caltech's Michael Brown, Chad Trujillo from the Gemini Observatory in Hawaii, and David Rabinowitz from Yale University.

The German team, led by Professor Frank Bertoldi from the University of Bonn and the Max Planck Institute for Radio Astronomy, used used a 30 metre (100ft) telescope in southern Spain and a "very sensitive heat sensor" to measure Xena's thermal emission, which was then used to calculate its diameter.

The figures state Xena has a diameter of 3,000km (1,864 miles), roughly 700km (435 miles) larger than Pluto. Accordingly, Professor Bertoldi told Reuters: "Since UB313 is decidedly larger than Pluto, it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status."

Bertoldi was referring to the polemic surrounding Xena's status as planet or otherwise. The International Astronomical Union will rule on the matter at some unspecified time, but the German observations make no contribution to the debate as they do not give any suggestion of Xena's composition.

Xena's 560 year orbit round the sun carries it between 5.6bn km (3.5bn miles) and 14.5bn km (9bn miles) from the star. It has a moon, Gabrielle, discovered last autumn, which added weight to the argument that Xena is indeed a planet. ®