10th planet in the solar system

Eris (pronounced /ˈɪərɪs/ EER-iss, or /ˈɛrɪs/ ERR-iss as in Greek Έρις),[10] formal designation 136199 Eris, is the largest known dwarf planet in the Solar System and the ninth-largest body known to orbit the Sun directly. It is approximately 2 500 kilometres in diameter and 27% more massive than Pluto.[11][8]

Eris was first spotted in 2003 by a Palomar Observatory-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 have found no evidence of further satellites. The current distance from the Sun is 96.7 AU, roughly three times that of Pluto. With the exception of some comets the pair are the most distant known natural objects in the Solar System.[2]

Because Eris is larger than Pluto, its discoverers and NASA called 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 is a "dwarf planet" along with Pluto, Ceres, Haumea and Makemake.[12]