The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto(1). It resides at present at 95.7 astronomical units (1 AU is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 UT. The event is consistent with a spherical shape for Eris, with radius 1,163 +/- 6 kilometres, density 2.52 +/- 0.05 grams per cm(3) and a high visible geometric albedo, p(V) = 0.96(-0.04)(+0.09). No nitrogen, argon or methane atmospheres are detected with surface pressure larger than similar to 1 nanobar, about 10,000 times more tenuous than Pluto's present atmosphere(2-5). As Pluto's radius is estimated(3-8) to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun.