An object some astronomers call the 10th planet is not as big as previously thought.

The round world, officially catalogued as 2003 UB313, is about 2,400 kilometers wide with an uncertainty of about 95 kilometers, according to new observations by the Hubble Space Telescope. Pluto is roughly 2,300 kilometers wide.

A study in February put the diameter of 2003 UB313 at roughly 3,000 kilometers.

Nicknamed "Xena," 2003 UB313 was discovered in 2005. By many accounts it could be considered a planet: It is round and orbits the Sun.

But because several other objects meet those criteria and also approach Pluto's size, astronomers have been wrangling for months over how to define the word "planet." It is not known if or when the International Astronomical Union, which rules on such things, will issue a decision. Members of an advisory board weighing the issue can't even agree on the parameters of a definition.

Since 2003 UB313 is 16 billion kilometers away and not even as wide as the United States, it showed up as just 1.5 pixels in Hubble's view. But that's enough to precisely make a size measurement, astronomers said.

The new size estimate suggests the object is brighter than had been thought. An object's brightness hints at its surface composition. In this case, a layer of methane frost might cover the world, astronomers speculate.

Comments: rbritt@space.com