Eris, in astronomy, the largest known dwarf planet. Eris, whose highly eccentric elliptical orbit ranges from 38 AU to 97 AU and is inclined more than 44°, is the largest known object of the Kuiper belt (see comet), with a diameter (1,445 mi/2,317 km) slightly larger than that of Pluto. Taking 560 earth years to circle the sun, Eris is believed to be composed of rock and ice. At aphelion (the most distant point from the sun in Eris's orbit), where the temperature is −405°C (−243°C), Eris's surface is covered with highly reflective frozen methane, which forms its atmosphere when it is closer to the sun and the surface temperature is warmer (−360°C/−218°C). Eris was discovered on Jan. 5, 2005, by astronomers Mike Brown, Chad Trujillo, and David Rabinowitz, using computer analysis of images taken two years earlier in a survey of the Kuiper belt. It was named for the Greek goddess of strife because its discovery was a catalyst for the reclassification of Pluto as a dwarf planet. Eris is known to have one natural satellite, Dysnomia, named for the daughter of the goddess Eris; the moon is estimated to have a diameter about an eighth that of Eris. Eris and its moon were nicknamed Xena and Gabrielle, respectively, before they were officially named.

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