2003 UB313, which may soon be named "Xena," was discovered in July 2005. It orbits the Sun at three times the distance of Pluto, but is unusually bright. Because of its unusual luminescence, scientists believed it was larger than Pluto - perhaps by as much as 30%. The latest analyses from Hubble images indicate that Xena is 2400 kilometers in diameter - only 5% larger than Pluto. Unlike Pluto, Xena seems to reflect virtually all the light that falls on its surface, making it the brightest planetary object in the solar system next to the Saturnine moon of Enceladus. Michael Brown of the California Institute of Technology calls it "unprecedented.

Richard Binzel of M.I.T. agrees. "Space is a dirty place. It's very hard to keep a surface bright and white - it requires some process to keep the surface fresh."

Brown suspects it may be due to the extreme elongation of Xena's 550-year orbit. When at perihelion (nearest the Sun), he explains, its atmosphere is in a gaseous state. When at aphelion (farthest from the Sun), the gas refreezes. Such a cycle of freeze and thaw might help to keep Xena's surface clean and bright.