Eris

**Eris** (minor-planet designation 136199 Eris) is the most massive and second-largest known dwarf planet in the Solar System. It is a trans-Neptunian object (TNO) in the scattered disk and has a high-eccentricity orbit. Eris was discovered in January 2005 by a Palomar Observatory–based team led by Mike Brown and 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 (counting 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 0.28% that of the Earth and 27% greater than that of Pluto, although Pluto is slightly larger by volume.

Eris has one large known moon, Dysnomia. In February 2016, Eris's distance from the Sun was 96.3 AU (14.41 billion km; 8.95 billion mi), more than three times that of Neptune or Pluto. With the exception of long-period comets, Eris and Dysnomia were the most distant known natural objects in the Solar System until the discovery of 2018 VG18 in 2018.
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, Pluto and Ceres are "dwarf planets", 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.

Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as Makemake and two days after Haumea, due in part to events that would later lead to controversy about Haumea. The search team had been systematically scanning for large outer Solar System bodies for several years, and had been involved in the discovery of several other large TNOs, including 50000 Quaoar, 90482 Orcus, and 90377 Sedna.
Routine observations were taken by the team on October 21, 2003, using the 1.2 m Samuel Oschin Schmidt telescope at Palomar Observatory, California, but the image of Eris was not discovered at that point due to its very slow motion across the sky: The team's automatic image-searching software excluded all objects moving at less than 1.5 arcseconds per hour to reduce the number of false positives returned. When Sedna was discovered in 2003, it was moving at 1.75 arcsec/h, and in light of that the team reanalyzed their old data with a lower limit on the angular motion, sorting through the previously excluded images by eye. In January 2005, the re-analysis revealed Eris's slow motion against the background stars.
Follow-up observations were then carried out to make a preliminary determination of Eris's orbit, which allowed the object's distance to be estimated. The team had planned to delay announcing their discoveries of the bright objects Eris and Makemake until further observations and calculations were complete, but announced them both on July 29 when the discovery of another large TNO they had been tracking—Haumea—was controversially announced on July 27 by a different team in Spain.
Precovery images of Eris have been identified back to September 3, 1954.
More observations released in October 2005 revealed that Eris has a moon, later named Dysnomia. Observations of Dysnomia's orbit permitted scientists to determine the mass of Eris, which in June 2007 was calculated to be (1.66±0.02)×1022 kg, 27%±2% greater than Pluto's.