The most distant known object in the Solar System, 2003 UB₃₁₃ (97 au from the Sun), was recently discovered near its aphelion¹. 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 UB₃₁₃, if adjusted to Pluto's distance, is greater than that of Pluto, which suggested that it might be larger than Pluto². 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 UB₃₁₃ 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 UB₃₁₃ 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 spectrum² causes a highly reflective icy surface.

1. Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2. Max-Planck-Institute for Radioastronomy, Auf dem Hügel 69, D-53121 Bonn, Germany
3. IRAM, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France

Correspondence to: F. Bertoldi^1,2 Correspondence and requests for materials should be addressed to F.B. (Email: bertoldi@astro.uni-bonn.de).

Received 7 October 2005; Accepted 23 November 2005

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