The trans-neptunian object UB313 is larger than Pluto

One over the nine The discovery of the trans-neptunian object 2003 UB 313 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. UB 313 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. The most distant known object in the Solar System, 2003 UB 313 (97 au from the Sun), was recently discovered near its aphelion 1 . Its high eccentricity and inclination to the ecliptic plane, along with its perihelion near the orbit of Neptune, identify it as a member of the ‘scattered disk’. This disk of bodies probably originates in the Kuiper belt objects, which orbit near the ecliptic plane in circular orbits between 30 and 50 au , and may include Pluto as a member. The optical brightness of 2003 UB 313 , if adjusted to Pluto's distance, is greater than that of Pluto, which suggested that it might be larger than Pluto 2 . The actual size, however, could not be determined from the optical measurements because the surface reflectivity (albedo) was unknown. Here we report observations of the thermal emission of 2003 UB 313 at a wavelength of 1.2 mm, which in combination with the measured optical brightness leads to a diameter of 3,000 ± 300 ± 100 km. Here the first error reflects measurement uncertainties, while the second derives from the unknown object orientation. This makes 2003 UB 313 the largest known trans-neptunian object, even larger than Pluto (2,300 km) 3 . The albedo is 0.60 ± 0.10 ± 0.05, which is strikingly similar to that of Pluto, suggesting that the methane seen in the optical spectrum 2 causes a highly reflective icy surface.