Eris (minor-planet designation: 136199 Eris) represents the most massive and second-largest recognized dwarf planet in the Solar System. Classified as a trans-Neptunian object (TNO) within the scattered disk, it exhibits a highly eccentric orbital path. Its discovery occurred in January 2005 by a team from Palomar Observatory, under the leadership of Mike Brown, with subsequent verification later that year. The object received its designation in September 2006, named after the Greco–Roman deity associated with strife and discord. Eris ranks as the ninth-most massive known object orbiting the Sun and the sixteenth-most massive within the Solar System, including moons. Furthermore, it holds the distinction of being the largest known Solar System object yet to be explored by a spacecraft. With a measured diameter of 2,326 ± 12 kilometers (1,445 ± 7 miles), Eris possesses a mass equivalent to 0.28% of Earth's and surpasses Pluto's mass by 27%, despite Pluto's marginally larger volume. The surface area of both Eris and Pluto is comparable to that of either Russia or South America.
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. 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 is orbited by one significant known moon, Dysnomia. As of February 2016, Eris maintained a heliocentric distance of 96.3 AU (14.41 billion km; 8.95 billion mi), exceeding the distances of Neptune or Pluto by more than threefold. Prior to the 2018 discoveries of 2018 AG37 and 2018 VG18, Eris and Dysnomia represented the most remote known natural objects in the Solar System, excluding long-period comets.
Initially, NASA characterized Eris as the Solar System's tenth planet, based on its apparent size exceeding that of Pluto. This observation, coupled with the potential discovery of other similarly sized objects, prompted the International Astronomical Union (IAU) to establish a formal definition for the term planet. According to the IAU definition approved on August 24, 2006, Eris, Pluto, and Ceres were reclassified as "dwarf planets," thereby reducing the count of recognized planets in the Solar System to eight, a number consistent with the pre-1930 era before Pluto's discovery. Subsequent observations of a stellar occultation by Eris in 2010 indicated it was marginally smaller than Pluto, whose mean diameter was measured by New Horizons as 2,377 ± 4 kilometers (1,477 ± 2 miles) in July 2015.
Discovery
Eris was identified by the collaborative team of Mike Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005, utilizing images originally captured on October 21, 2003. The announcement of its discovery occurred on July 29, 2005, coinciding with that of Makemake and two days following the announcement of Haumea, partly influenced by subsequent controversies surrounding Haumea. For several years, this research team had systematically surveyed for substantial outer Solar System bodies, contributing to the discovery of numerous other large TNOs, such as the dwarf planets Quaoar, Orcus, and Sedna.
On October 21, 2003, the team conducted routine observations using the 1.2-meter Samuel Oschin Schmidt telescope at Palomar Observatory, California; however, Eris's image was not initially detected due to its exceptionally slow apparent motion across the celestial sphere. The automated image-searching software employed by the team filtered out objects exhibiting angular motion below 1.5 arcseconds per hour, a measure implemented to minimize false positive detections. Following Sedna's discovery in 2003, which displayed a motion of 1.75 arcseconds per hour, the team subsequently re-evaluated their archived data, applying a reduced threshold for angular motion and manually inspecting previously excluded images. This re-analysis, conducted in January 2005, ultimately unveiled Eris's subtle orbital progression against the stellar background.
Subsequent follow-up observations were performed to establish a preliminary orbital determination for Eris, thereby enabling an estimation of the object's distance. The team had initially intended to postpone the announcement of their discoveries of the luminous objects Eris and Makemake until comprehensive observations and calculations were finalized. However, both were announced on July 29, prompted by the controversial announcement on July 27 by a separate Spanish team regarding Haumea, another large TNO that the original team had also been tracking.
Precovery images of Eris have been identified, dating back to September 3, 1954.
Further observations, published in October 2005, disclosed the presence of a moon orbiting Eris, subsequently designated Dysnomia. Analysis of Dysnomia's orbital characteristics enabled scientists to ascertain Eris's mass, which was computed in June 2007 to be (1.66±0.02)×1022 kg, representing an increase of 27%±2% over Pluto's mass.
Name
Eris derives its appellation from the Greek goddess Eris (Greek Ἔρις), who embodies strife and discord. The Caltech research team submitted this name on September 6, 2006, and it received official assignment on September 13, 2006. This formal naming followed an extended period during which the celestial body was provisionally identified as 2003 UB313, a designation automatically conferred by the IAU in accordance with their established protocols for minor planets.
The appellation Eris exhibits two prevalent pronunciations, distinguished by either a "long" or a "short" e vowel sound, mirroring the phonetic variations observed in the word era. In English, the more frequently encountered pronunciation, adopted by individuals such as Brown and his research group, is , characterized by disyllabic laxing and a short e. Conversely, the traditional English pronunciation for the goddess is , featuring a long e.
The Greek and Latin oblique stem for this name is Erid-, evident in terms like the Italian Eride and the Russian Эрида Erida. Consequently, the corresponding adjectival form in English is Eridian .
Xena
The classification ambiguity of the celestial body, specifically whether it constituted a planet or a minor planet, necessitated a delay in its official naming until after the International Astronomical Union's (IAU) ruling on August 24, 2006, given the distinct nomenclature protocols for each category. During this interim, the object gained widespread public recognition under the informal designation Xena. This internal nickname, adopted by the discovery team, drew inspiration from the protagonist of the television series Xena: Warrior Princess. Reports indicate that the discovery team had reserved the moniker "Xena" for the initial celestial body they identified that surpassed Pluto in size. As stated by 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 indicated in an interview that the naming procedure encountered delays:
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.
Choosing an Official Name
Science writer Govert Schilling reported that Brown initially intended to name the object "Lila," referencing a Hindu mythological concept that portrays the cosmos as the result of a divine game played by Brahman. This proposed name shared its pronunciation with "Lilah," Brown's infant daughter. Brown exercised caution, aiming to avoid publicizing the name before its official acceptance, a protocol he had previously violated with Sedna a year prior, incurring significant criticism. Nevertheless, the Sedna designation faced no objections beyond the procedural breach, and no alternative names were put forth.
Brown designated the URL of his personal webpage, which announced the discovery, as /~mbrown/planetlila. Amidst the subsequent turmoil surrounding the Haumea discovery controversy, he neglected to modify this address. To avoid further alienating his astronomical colleagues, he subsequently explained that the webpage's name honored his daughter and consequently withdrew "Lila" from consideration for the object's official designation.
Brown also contemplated naming the object Persephone, after the consort of the deity Pluto. This name had appeared multiple times in science fiction narratives concerning planets and enjoyed considerable public favor, evidenced by its decisive victory in a poll conducted by New Scientist magazine. (Notably, "Xena," despite its informal status as a nickname, secured fourth place.) However, this selection proved unfeasible due to the prior existence of a minor planet, 399 Persephone, already bearing that name, and Eris was being formally classified as a minor planet.
The discovery team formally proposed the name Eris on September 6, 2006. Brown determined that, given the object's prolonged consideration as a planet, it warranted a name derived from Greek or Roman mythology, consistent with other planets. Since asteroids had already claimed the vast majority of Graeco-Roman names, Eris, whom Brown characterized as his preferred goddess, had fortuitously remained available. In 2006, Brown remarked, "Eris caused strife and discord by causing quarrels among people, and that's what this one has done too." The International Astronomical Union (IAU) officially accepted the name on September 13, 2006.
While the use of planetary symbols is generally discouraged within astronomy, NASA has adopted the Hand of Eris, ⟨⟩ (U+2BF0), for Eris. This particular symbol originates from Discordianism, a religious movement centered on the goddess Eris. The Sternberg Astronomical Institute at Moscow State University has employed (U+24C0), which is presumed to be either the "all rights reversed" symbol from the Principia Discordia or a simplified representation of the Apple of Discord, inscribed with the Greek word Kallisti, . This latter symbol had been initially suggested for the dwarf planet on a Discordian discussion board, which ultimately settled on ⟨⟩. Most astrologers utilize the Hand of Eris, although alternative symbols, such as ⟨⟩ (U+2BF1), are occasionally observed.
Classification
Eris is categorized as a trans-Neptunian dwarf planet. More specifically, its orbital characteristics classify it as a scattered-disk object (SDO), which is a trans-Neptunian object (TNO) that has been gravitationally perturbed from the Kuiper belt into more distant and atypical orbits during the Solar System's formation. Although its pronounced orbital inclination is uncommon among known SDOs, theoretical models indicate that objects originating near the inner edge of the Kuiper belt were scattered into orbits with higher inclinations compared to those from the outer belt.
Initially believed to be larger than Pluto, Eris was widely referred to as the "tenth planet" by NASA and in media reports following its discovery. In response to the ambiguity surrounding its status and the ongoing debate regarding Pluto's planetary classification, the IAU tasked a group of astronomers with formulating a sufficiently precise definition of the term planet to resolve the issue. This definition, titled the IAU's Definition of a Planet in the Solar System, was adopted on August 24, 2006. At that time, both Eris and Pluto were reclassified as dwarf planets, a distinct category from the newly defined planet. Brown has since expressed his approval of this classification. Subsequently, the IAU incorporated Eris into its Minor Planet Catalogue, assigning it the designation (136199) Eris.
Orbit
Eris completes an orbit around the Sun every 561 years. Its maximum distance from the Sun (aphelion) reaches 97.7 astronomical units (AU), while its closest approach (perihelion) is 38.4 AU. The accuracy of the perihelion time, defined at a chosen epoch using an unperturbed two-body solution, diminishes as the epoch deviates from the actual perihelion date. Precise prediction of perihelion time necessitates numerical integration. Numerical integration performed by JPL Horizons indicates that Eris reached perihelion around 1699, aphelion around 1977, and is projected to return to perihelion on December 6, 2257. Unlike the eight major planets, whose orbits lie approximately within the same plane as Earth's, Eris's orbit is highly inclined, tilted at an angle of approximately 44 degrees to the ecliptic. Upon its discovery, Eris and its moon were the most distant known objects in the Solar System, excluding long-period comets and space probes. This distinction was maintained until the discovery of 2018 VG18 in 2018.
The Eridian orbit exhibits high eccentricity, bringing Eris to within 37.9 AU of the Sun, which is characteristic of perihelion distances for scattered objects. This distance places it within Pluto's orbit, yet it remains safely beyond direct gravitational interaction with Neptune (~37 AU). In contrast, Pluto, along with other plutinos, follows a less inclined and less eccentric orbit and, due to orbital resonance, can cross Neptune's path. Approximately 800 years from now, Eris will temporarily be closer to the Sun than Pluto.
In 2007, Eris exhibited an apparent magnitude of 18.7, rendering it sufficiently luminous for detection by certain amateur telescopes. Under optimal conditions, a 200-millimeter (7.9-inch) telescope equipped with a charge-coupled device (CCD) can identify Eris. Its delayed discovery until 2005 is attributable to its pronounced orbital inclination, as surveys for substantial outer Solar System objects typically focus on the ecliptic plane, where the majority of celestial bodies reside.
Due to its significantly inclined orbit, Eris traverses a limited number of constellations within the traditional Zodiac. In 2026, it is situated within the constellation Cetus. Historically, it resided in Sculptor from 1876 to 1929 and in Phoenix approximately from 1840 to 1875. Projections indicate that in 2036, it will enter Pisces, remaining there until 2065, at which point it will transition into Aries. Subsequently, its trajectory will shift towards the northern celestial hemisphere, entering Perseus in 2128 and Camelopardalis in 2173, where it is anticipated to attain its maximum northern declination.
Dimensions, Mass, and Density
In November 2010, Eris underwent one of the most remote stellar occultations observed from Earth to date. Initial data derived from this phenomenon challenged prior estimations of its size. The research teams subsequently published their definitive findings from the occultation in October 2011, reporting an estimated diameter of 2326±12 km.
Consequently, Eris is marginally smaller than Pluto, which measures 2376.6±1.6 km in diameter, despite Eris possessing greater mass. Furthermore, these observations suggest a geometric albedo of 0.96. The elevated albedo is hypothesized to result from the continuous replenishment of surface ices, driven by temperature variations as Eris traverses its eccentric orbit, moving both nearer to and farther from the Sun.
Eris's mass can be determined with significantly higher precision. Utilizing the then-accepted orbital period of Dysnomia, 15.774 days, Eris is calculated to be 27% more massive than Pluto. Incorporating the 2011 occultation data, Eris exhibits a density of 2.52±0.07 g/cm§56§, which is considerably greater than Pluto's, implying a predominant composition of rocky materials.
Theoretical models of internal heating, primarily through radioactive decay, propose the potential existence of a subsurface liquid water ocean within Eris, situated at the mantle–core interface. Furthermore, tidal heating exerted on Eris by its moon, Dysnomia, could contribute to the sustained presence of such a subsurface ocean. Subsequent investigations have concluded that Eris, alongside Pluto and Makemake, may host active subsurface oceans and exhibit ongoing geothermal activity.
In July 2015, following approximately a decade during which Eris was considered the ninth-largest known object in direct solar orbit, detailed imagery acquired by the New Horizons mission revealed Pluto's volume to be marginally greater than Eris's. Consequently, Eris is now classified as the tenth-largest known object directly orbiting the Sun by volume, while retaining its position as the ninth-largest by mass.
Surface and Atmosphere
Subsequent to its initial identification, the discovery team conducted spectroscopic observations of Eris on January 25, 2005, utilizing the 8-meter Gemini North Telescope in Hawaii. Infrared analysis of the object's emitted light indicated the presence of methane ice, suggesting a surface composition analogous to Pluto's—then the sole known Trans-Neptunian Object (TNO) with surface methane—and Neptune's moon Triton, which similarly exhibits surface methane. In 2022, near-infrared spectroscopy performed by the James Webb Space Telescope (JWST) detected deuterated methane ice on Eris's surface, present in lower abundances compared to those found in Jupiter-family comets such as 67P/Churyumov–Gerasimenko. This relatively low deuterium abundance implies that Eris's methane is not primordial but rather may have originated from subsurface geochemical processes. The JWST also identified significant quantities of nitrogen ice on Eris, which is presumed to have formed through subsurface processes analogous to those hypothesized for Eris's non-primordial methane. Volumetrically, the abundance of nitrogen ice on Eris is estimated at one-third that of methane.
In stark contrast to the reddish, variegated surfaces observed on Pluto and Triton, Eris presents an almost uniformly white appearance. The distinctive reddish hue of Pluto is attributed to surface deposits of tholins; these darkened areas, characterized by a lower albedo, experience elevated temperatures, facilitating the sublimation of methane deposits. Conversely, Eris's greater distance from the Sun permits methane condensation across its surface, even in regions with reduced albedo. This uniform methane condensation effectively mitigates albedo variations and would obscure any underlying red tholin deposits. Such a methane sublimation-condensation cycle might generate bladed terrain on Eris, analogous to formations observed on Pluto. An alternative hypothesis suggests that Eris's surface undergoes renewal via radiogenic convection within a global methane and nitrogen ice glacier, akin to Pluto's Sputnik Planitia. Spectroscopic data from the James Webb Space Telescope (JWST) corroborate the notion of continuous surface refreshing on Eris, given the absence of detectable ethane, a known byproduct of radiolyzed methane.
Eris's highly eccentric and distant orbit results in estimated surface temperatures ranging from approximately 30 to 56 K (−243.2 to −217.2 °C; −405.7 to −358.9 °F). Despite potentially orbiting up to three times farther from the Sun than Pluto, Eris's perihelion allows for sufficient warming of some surface ices to sublime, potentially forming a transient atmosphere. The detection of highly volatile methane and nitrogen suggests two primary scenarios: either Eris has perpetually occupied the frigid outer Solar System, where these ices can endure, or the celestial body possesses an internal mechanism to replenish atmospheric gases lost to space. This finding diverges from observations of Haumea, another trans-Neptunian object, which indicate the presence of water ice but a notable absence of methane.
Rotational Characteristics
The uniform surface of Eris results in minimal brightness variations during its rotation, thereby complicating the precise determination of its rotational period. Extended, high-precision monitoring of Eris's luminosity has revealed that it is tidally locked with its moon, Dysnomia, exhibiting a rotational period precisely synchronized with Dysnomia's 15.78 Earth-day orbital period. Furthermore, Dysnomia is similarly tidally locked to Eris, establishing the Eris–Dysnomia system as only the second confirmed instance of double-synchronous rotation, following the Pluto–Charon binary. Prior attempts to measure Eris's rotation period yielded highly uncertain values, spanning from tens of hours to several days, primarily due to inadequate long-term observational data. While Eris's axial tilt remains unmeasured, it is plausibly inferred to align with Dysnomia's orbital inclination, estimated at approximately 78 degrees relative to the ecliptic plane. Under this assumption, a significant portion of Eris's northern hemisphere would receive solar illumination, with 30% of that hemisphere experiencing perpetual daylight in 2018.
Natural Satellite
In 2005, researchers utilizing the newly operational laser guide star adaptive optics system at the Keck telescopes in Hawaii conducted observations of the four most luminous trans-Neptunian objects: Pluto, Makemake, Haumea, and Eris. Imagery acquired on September 10 definitively identified a moon orbiting Eris. Consistent with the informal designation "Xena" already applied to Eris, Brown's research group provisionally named the moon "Gabrielle," referencing the television warrior princess's companion. Upon Eris's official nomenclature by the International Astronomical Union (IAU), its moon was formally designated Dysnomia, named after the Greek goddess of lawlessness and daughter of Eris. Brown stated that the name was chosen due to its phonetic resemblance to his wife's name, Diane. Furthermore, the name inadvertently maintains an indirect allusion to Eris's former informal appellation, Xena, a character famously portrayed by actress Lucy Lawless, although this connection was not deliberately intended.
Spacecraft Exploration
In May 2020, the outbound New Horizons spacecraft conducted distant observations of Eris during its extended mission, subsequent to its successful Pluto flyby in 2015. Despite New Horizons being at a greater distance from Eris (112 AU) than Eris was from Earth (96 AU), the spacecraft's unparalleled position within the Kuiper Belt facilitated observations of Eris at high phase angles, which are otherwise inaccessible from terrestrial vantage points. This enabled the characterization of the Eridian surface's light scattering properties and phase curve behavior.
During the 2010s, several investigations were conducted regarding subsequent missions to explore the Kuiper Belt, with Eris being considered a potential target. Projections indicated that a flyby mission to Eris, utilizing a Jupiter gravity-assist, would require 24.66 years, assuming launch dates of April 3, 2032, or April 7, 2044. Upon the spacecraft's arrival, Eris would be situated at a distance of either 92.03 or 90.19 AU from the Sun.
Explanatory notes
References
- Michael Brown's webpages about Eris and Dysnomia
- 3D orbit visualization from NASA JPL
- Keck observatory page about the discovery of Dysnomia