Eris (IPA: /ˈɛɹɪs, ˈɪɹɪs/), also designated 136199 Eris and formerly 2003 UB313 (see minor planet names), is the largest known dwarf planet in the solar system and the ninth largest body orbiting the Sun directly. It is between 2,400 and 3,000 kilometres (1,490 to 1,860 miles) in diameter and 27% more massive than Pluto.
Eris was discovered in 2005 by a Mount Palomar-based team led by Mike Brown. It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc. Eris has one moon, named Dysnomia; recent observations found no evidence of further satellites. Their current distance from the Sun is some 97 AU, or roughly three times that of Pluto. With the exception of some comets, the pair are currently the most distant known bodies in the Solar System.
Eris’ size resulted in its discoverers and NASA labelling it the solar system's tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term "planet" for the first time. Under a new definition approved on August 24, 2006, Eris was designated a "dwarf planet" along with Pluto and Ceres.