Eris, minor-planet designation 136199 Eris, is the most massive known dwarf planet in the Solar System, and is known to orbit the Sun directly. Eris is estimated to be 27% more massive than Pluto, or about 0.27% of the Earth's mass.

Eris was discovered in 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year.

As Eris appeared to be larger then Pluto, NASA designated Eris the tenth planet in the Solar System. 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. As of August 26th, 2006, Eris has been designated a 'Dwarf planet' along with other planets such as Pluto, Ceres, Haumea and Makemake.