Mike Brown of the California Institute of Technology, Chad Trujillo of the Gemini Observatory in Hawaii and David Rabinowitz of Yale University discovered 2003 UB313 with the Samuel Oschin telescope at the Palomar Observatory in California. They first photographed the new planet in October 2003 but it was so far away that they did not actually realize it might be a planet until they reanalysed their data in January this year.

2003 UB313 lies in a vast band of small, icy bodies beyond Neptune called the Kuiper belt. The new planet seems to be typical of Kuiper-belt objects but is much bigger -- between about 2300 and 3200 km wide. Pluto is 2390 km across. "Its sheer size in relation to the nine known planets means it can only be classified as a planet itself," says Brown.

Brown and colleagues have already obtained near-infrared spectra of the new planet with the Gemini North Telescope, which show signatures of methane ice very similar to the spectrum of Pluto. The presence of methane ice indicates a pristine surface that has not been significantly heated since the solar system was formed about 4.5 billion years ago. The discovery suggests that other "transneptunian" objects may be hiding undiscovered in the far reaches of the Solar System. Last year Brown and colleagues discovered an object called "Sedna", which was previously the most distant object in the Solar System.

Meanwhile, astronomers at the Sierra Nevada Observatory in Spain have discovered a second large Kuiper-belt object called "2003 EL61". This object is about two-thirds the size of Pluto and is about 51 astronomical units (AU) away from the Sun, where 1 AU is the average distance between the Earth and the Sun. Pluto, by comparison, is about 39 AU from the Sun.