Largest of the three known dwarf planets, orbiting beyond Neptune. Eris was discovered in 2005. It belongs to the disc of small bodies beyond Neptune called the Kuiper belt.
When its diameter was calculated as approximately 2,600 km/1,615 mi, slightly greater than that of Pluto, then classified as the ninth planet, the question arose of whether Eris should be classified as a planet. This prompted the International Astronomical Union (IAU) to adjudicate the question of the definition of a planet. As a result, the new body was classified as a dwarf planet, while Pluto was controversially demoted to the same status. Until then the new body had been designated ‘Xena’. It was now given the appropriate name Eris, after the Greek goddess of discord. Eris is known to have a satellite, which is called Dysnomia, after a Greek personification of lawlessness. The latest measurements of the diameter of Eris give a value of 2,326 +− 12 km/1,445 +− 7 mi – very close to that of Pluto.
Eris is composed of rock and ice. It moves in an orbit that is tilted at more than 44° to the ecliptic, the plane in which, or close to which, the other planets move. Eris approaches to 37.8 astronomical units (AU) from the Sun. (One AU is the mean Earth–Sun distance of 150 million km/93 million mi.) However, the orbit of Eris is highly eccentric (non-circular): at its furthest, Eris is 97.6 AU from the Sun. When it is furthest from the Sun, frozen methane covers its surface. When Eris is closest to the Sun, 280 years later, the methane evaporates to form an atmosphere of methane. Even here the surface temperature is −360°F/−218°C.
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