2003 UB313 is a trans-Neptunian object (TNO) believed to be larger than the planet Pluto[1]. It has been described as our solar system's tenth planet by its Mount Palomar-based discovery team, NASA, and some media outlets. The International Astronomical Union (IAU) is scheduled to publish the definition of the term "planet" in early September 2006, which will determine whether 2003 UB313 is classified as a planet. [2]

2003 UB313 orbits the Sun in a region of space known as the scattered disc accompanied by at least one moon; the pair are currently the Solar System's most distant known objects. Recent thermal observations by astrophysicists at IRAM have determined the object's diameter to be in the range of 2400 to 3400 km and its albedo to be 40-90% [3] with certainty of 70%. Brown's team published their size estimate from Hubble's picture and it is 2400 ± 100 km, and meanwhile has detected methane ice on the object's surface. These observations suggest that 2003 UB313 is more similar to Pluto than any previously known body.

A ruling on what to name 2003 UB313 is currently being delayed pending a decision on whether it qualifies as a planet or not. 2003 UB313's discoverers have confidentially submitted potential names to the IAU, who oversee astronomical naming conventions. Claims that 2003 UB313 has been officially named 'Xena' or 'Lila' are incorrect; 'Xena' is an informal codename used by its discoverers among themselves, and 'Lila' is a name in the address of the website where the object was announced, after the newly-born daughter of one of the discoverers. Neither name was submitted to the IAU.