Eris (minor planet designation 136199 Eris) is the most massive and second-largest known dwarf planet in the Solar System. Eris is a trans-Neptunian object (TNO), has a high-eccentricity orbit, and is a member of the scattered disk. Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its discovery was verified later that year. In September 2006 it was named after the Greco-Roman goddess of strife and discord. Eris is the ninth-most massive known object orbiting the Sun, and the sixteenth-most massive overall in the Solar System (including moons). It is also the largest object that has not been visited by a spacecraft. Eris has been measured at 2,326 ± 12 kilometers (1,445 ± 7 mi) in diameter. Its mass is 0.27 percent that of the Earth and 127 percent that of dwarf planet Pluto, though Pluto is slightly larger by volume. As Eris orbits the Sun, it completes one rotation every 25.9 hours, making its day length similar to Earth's. However, other sources disagree on the rotation period.

It has one large known moon, Dysnomia. In February 2016, its distance from the Sun was 96.3 astronomical units (1.441×1010 km; 8.95×109 mi), roughly three times that of Pluto. With the exception of some long-period comets, until 2018 VG18 was discovered on December 17, 2018, Eris and Dysnomia were the most distant known natural objects in the Solar System.

Because Eris appeared to be larger than Pluto, NASA initially described it as the Solar System's tenth planet. This, along with the prospect of other objects of similar size 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, thereby reducing the number of known planets in the Solar System to eight, the same as before Pluto's discovery in 1930. Observations of a stellar occultation by Eris in 2010 showed that it was very slightly smaller than Pluto, which was measured by New Horizons as 2,377 ± 4 kilometers (1,477 ± 2 mi) in July 2015.

 

Note:   The above text is excerpted from the Wikipedia article Eris (dwarf planet), which has been released under the GNU Free Documentation License.