Further observations of an object dubbed 2003 UB313, which lies beyond Neptune, show that its diameter is around 3,100 kilometres. This makes it larger than Pluto, the smallest ‘traditional’ Solar System planet. The discovery of the trans-neptunian object 2003 UB313 in July 2005 has rekindled the debate over which objects should be called planets. Popularly but not officially known as the ‘tenth planet’, its optical brightness suggests it is larger than Pluto, but without knowing its surface reflectivity (albedo) it is not possible to determine size from optical data. But now a pretty accurate measure of its size has be made, based on thermal emission data obtained using the IRAM 30-metre radio telescope at Pico Veleta, Spain. UB313 is about 3,000 km in diameter, compared to Pluto's 2,000 km. Its albedo is similar to Pluto's, consistent with a highly reflective icy surface rich in methane.