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.

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.

Observations of a stellar occultation by Eris in 2010 showed that it was slightly smaller than Pluto,[27][28] which was measured by New Horizons as having a mean diameter of 2,377 ± 4 kilometres (1,477 ± 2 mi) in July 2015.

[29][30] Eris was discovered by the team of Mike Brown, Chad Trujillo, and David Rabinowitz[2] on January 5, 2005, from images taken on October 21, 2003.

[33] 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.

[31] 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.

[31] 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.

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,[23] 27%±2% greater than Pluto's.

[42]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.

[2] The name had been used several times for planets in science fiction[44] and was popular with the public, having handily won a poll conducted by New Scientist magazine.

[50] The Sternberg Astronomical Institute at Moscow State University has used (U+24C0), presumably either the "all rights reversed" symbol of the Principia Discordia or a simplification of the Apple of Discord inscribed with the Greek word Kallisti, , which had been suggested as a symbol for the dwarf planet on the Discordian discussion board that eventually settled on ⟨⟩.

[53] Because Eris was initially thought to be larger than Pluto, it was described as the "tenth planet" by NASA and in media reports of its discovery.

[6] When discovered, Eris and its moon were the most distant known objects in the Solar System, apart from long-period comets and space probes.

[7] The Eridian orbit is highly eccentric, and brings Eris to within 37.9 AU of the Sun, a typical perihelion for scattered objects.

[f] The reason it had not been noticed until now is its steep orbital inclination; searches for large outer Solar System objects tend to concentrate on the ecliptic plane, where most bodies are found.

It is speculated that the high albedo is due to the surface ices being replenished because of temperature fluctuations as Eris's eccentric orbit takes it closer and farther from the Sun.

Using the 2011 occultation results, Eris has a density of 2.52±0.07 g/cm3,[g] substantially denser than Pluto, and thus must be composed largely of rocky materials.

[12] Models of internal heating via radioactive decay suggest that Eris could have a subsurface ocean of liquid water at the mantle–core boundary.

[72] Eris is now understood to be the tenth-largest known object to directly orbit the Sun by volume, but remains the ninth-largest by mass.

The discovery team followed up their initial identification of Eris with spectroscopic observations made at the 8 m Gemini North Telescope in Hawaii on January 25, 2005.

[73] In 2022, near-infrared spectroscopy of Eris by the James Webb Space Telescope (JWST) revealed the presence of deuterated methane ice on its surface, at abundances lower than those in Jupiter-family comets like 67P/Churyumov–Gerasimenko.

[74] Eris's comparatively low deuterium abundance suggests that its methane is not primordial and instead may have been produced from subsurface geochemical processes.

[75][74] Alternatively, Eris's surface could be refreshed through radiogenic convection of a global methane and nitrogen ice glacier, similar to Pluto's Sputnik Planitia.

[2] Even though Eris can be up to three times farther from the Sun than Pluto, it approaches close enough that some of the ices on the surface might warm enough to sublime to form an atmosphere.

[15] Dysnomia is also tidally locked to Eris, which makes the Eris–Dysnomia system the second known case of double-synchronous rotation, after Pluto and Charon.

[78][79][80] The axial tilt of Eris has not been measured,[14] but it can be reasonably assumed that it is the same as Dysnomia's orbital inclination, which would be about 78 degrees with respect to the ecliptic.

[82] Eris was observed from afar by the outbound New Horizons spacecraft in May 2020, as part of its extended mission following its successful Pluto flyby in 2015.

[17] Although Eris was farther from New Horizons (112 AU) than it was from Earth (96 AU), the spacecraft's unique vantage point inside the Kuiper belt permitted observations of Eris at high phase angles that are otherwise unobtainable from Earth, enabling the determination of the light scattering properties and phase curve behavior of the Eridian surface.