Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly. It is estimated to be 2326 (±12) km in diameter, and 27% more massive than Pluto, or about 0.27% of the Earth's mass.
Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. It is a trans-Neptunian object (TNO) and a member of a high-eccentricity population known as the scattered disc. It has one known moon, Dysnomia. As of 2011, its distance from the Sun is 96.6 AU, roughly three times that of Pluto. With the exception of some comets, Eris and Dysnomia are currently the most distant known natural objects in the Solar System.
Because Eris appeared to be larger than Pluto, its discoverers and NASA initially described it as 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 the IAU definition approved on August 24, 2006, Eris is a "dwarf planet", along with objects such as Pluto, Ceres, Haumea and Makemake.
In 2010, preliminary results from observations of a stellar occultation by Eris on November 6 suggested that its diameter may be only 2,326 km, which would make it essentially the same size as Pluto. Given the error bars in the different size estimates, it is currently uncertain whether Eris or Pluto has the larger diameter. Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km.