Eris (minor-planet number 136199) 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. It was named in September 2006 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 in the Solar System (counting moons). It is also the largest-known object in the Solar System that has not been visited by a spacecraft. Eris has been measured at 2,326 ± 12 kilometres (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. Both Eris and Pluto have a surface area that is comparable to that of Russia or South America.
Eris has one large confirmed moon, Dysnomia, and it might have another much larger moon. 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 AG37 and 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 slightly smaller than Pluto, which was measured by New Horizons as having a mean diameter of 2,377 ± 2 kilometres (1,477 ± 1 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 the dwarf planets Quaoar, Orcus and 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 orbital 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.
Eris is named after the Greek goddess Eris (Greek Ἔρις), a personification of strife and discord. The name was proposed by the Caltech team on September 6, 2006, and it was assigned on September 13, 2006, following an unusually long period in which the object was known by the provisional designation 2003 UB313, which was granted automatically by the IAU under their naming protocols for minor planets.
The name Eris has two competing pronunciations, with a "long" or with a "short" e, analogous to the two competing pronunciations of the word era. Perhaps the more common form in English, used i.a. by Brown and his students, is with disyllabic laxing and a short e. However, the classical English pronunciation of the goddess is , with a long e.
The Greek and Latin oblique stem of the name is Erid-, as can be seen in Italian Eride and Russian Эрида Erida, so the adjective in English is Eridian .
Due to uncertainty over whether the object would be classified as a planet or a minor planet, because varying nomenclature procedures apply to these classes of objects, the decision on what to name the object had to wait until after the August 24, 2006, IAU ruling. For a period of time, the object became known to the wider public as Xena. "Xena" was an informal name used internally by the discovery team, inspired by the title character of the television series Xena: Warrior Princess. The discovery team had reportedly saved the nickname "Xena" for the first body they discovered that was larger than Pluto. According to Brown,
We chose it since it started with an X (planet "X"), it sounds mythological ... and we've been working to get more female deities out there (e.g. Sedna). Also, at the time, the TV show was still on TV, which shows you how long we've been searching!
Brown said in an interview that the naming process was stalled:
One reporter [Ken Chang] called me up from The New York Times who happened to have been a friend of mine from college, [and] ... asked me, "What's the name you guys proposed?" and I said, "Well, I'm not going to tell." And he said, "Well, what do you guys call it when you're just talking amongst yourselves?" ... As far as I remember this was the only time I told anybody this in the press, and then it got everywhere, which I only sorta felt bad about; I kinda like the name.
According to science writer Govert Schilling, Brown initially wanted to call the object "Lila", after a concept in Hindu mythology that described the cosmos as the outcome of a game played by Brahman. The name would be pronounced the same as "Lilah", the name of Brown's newborn daughter. Brown was mindful of not making his name public before it had been officially accepted. He had done so with Sedna a year previously, and had been heavily criticized. However, no objection was raised to the Sedna name other than the breach of protocol, and no competing names were suggested.
He listed the address of his personal web page announcing the discovery as /~mbrown/planetlila and in the chaos following the controversy over the discovery of Haumea, forgot to change it. Rather than needlessly anger more of his fellow astronomers, he simply said that the webpage had been named for his daughter and dropped "Lila" from consideration.
Brown had also considered Persephone, the wife of the god Pluto, as a name for the object. The name had been used several times for planets in science fiction and was popular with the public, having handily won a poll conducted by New Scientist magazine. ("Xena", despite only being a nickname, came fourth.) This choice was not possible because there was already a minor planet with that name, 399 Persephone, and Eris was being named as a minor planet.