Eris is the dwarf planet discovered in 2005 that helped prompt Pluto’s reclassification. Its discovery revealed a distant object initially thought to be at least as large as Pluto, making the definition of “planet” impossible to avoid.
A team led by Mike Brown, Chad Trujillo, and David Rabinowitz identified Eris in images taken in 2003 and announced the discovery in July 2005. News reports and NASA initially referred to it as the possible “tenth planet.” Later measurements showed that Eris is more massive than Pluto, although Pluto is slightly larger by volume.
The International Astronomical Union adopted a formal planet definition on August 24, 2006. Under that definition, a planet must orbit the Sun, be rounded by its own gravity, and clear its orbital neighborhood. Pluto and Eris meet the first two conditions but not the third, so both were classified as dwarf planets.
Eris is a trans-Neptunian object in the scattered disk and has one known moon, Dysnomia. Ceres, Haumea, and Makemake are also dwarf planets, but Eris was the discovery most directly tied to the Pluto debate.