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Eris (IPA: /ˈɛrɪs, ˈɪərɪs/), formally 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 known to orbit the Sun directly. It is between 2,400 and 3,000 kilometres in diameter and 27% more massive than Pluto.

Eris was first spotted in 2003 by a Mount Palomar-based team led by Mike Brown (but not identified until 2005). 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, 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.

Discovery

Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as two other large TNOs, (136108) 2003 EL₆₁ and (136472) 2005 FY₉. The search team has been systematically scanning for large outer solar system bodies for several years, and had previously been involved in the discovery of several other very large TNOs, including 50000 Quaoar, 90482 Orcus, and 90377 Sedna.

Routine observations were taken by the team on October 21, 2003, using the 1200 mm Samuel Oschin reflecting telescope at Mount Palomar Observatory, California, but the object captured on the images was not discovered at that point due to its very slow motion across the sky: the team's automatic image-searching software excluded all objects moving at less than 1.5 arcseconds per hour to reduce the number of false positives returned. When Sedna was discovered, it was moving at 1.75 arcsec/h, and in light of that the team reanalyzed their old data with a lower limit on the angular motion, sorting through the previously excluded images by eye. In January 2005, the re-analysis revealed Eris' slow motion against the background stars.

Read more at Wikipedia.org