New Scientist SPACE - Breaking News - 'Tenth planet' is bigger than Pluto

"The chilly world dubbed Xena on the outskirts of the solar system has as at least as much claim to be a planet as Pluto, according to a new study confirming that the 'tenth planet' is by far the larger of the two.

Astronomers first spotted Xena, known more formally as UB 313, in 2003, but the discovery was not made public until July 2005. By then they realised it was amazingly distant – at times, about three times as far from the Sun as Pluto is.

From its brightness and distance, astronomers estimated that it must be slightly larger than Pluto – assuming that, like Pluto, it reflects just over half the sunlight that falls onto it."