Eris (dwarf planet)

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 and two days after , 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 , , and . 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 , greater than Pluto's. == Name ==

Eris is named after the Greek goddess Eris (Greek ), a personification of strife and discord. following an unusually long period in which the object was known by the provisional designation , 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. as can be seen in Italian Eride and Russian Эрида Erida, so the adjective in English is Eridian . 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. The name was accepted by the IAU on September 13, 2006. This symbol was taken from Discordianism, a religion concerned with the goddess Eris. Most astrologers use the Hand of Eris, though other symbols are occasionally seen, such as (U+2BF1). == Classification ==

Eris is a trans-Neptunian dwarf planet. Its orbital characteristics more specifically categorize it as a scattered-disk object (SDO), or a TNO that has been "scattered" from the Kuiper belt into more-distant and unusual orbits following gravitational interactions with Neptune as the Solar System was forming. Although its high orbital inclination is unusual among the known SDOs, theoretical models suggest that objects that were originally near the inner edge of the Kuiper belt were scattered into orbits with higher inclinations than objects from the outer belt. 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. In response to the uncertainty over its status, and because of ongoing debate over whether Pluto should be classified as a planet, the IAU delegated a group of astronomers to develop a sufficiently precise definition of the term planet to decide the issue. This was announced as the IAU's Definition of a Planet in the Solar System, adopted on August 24, 2006. At this time, both Eris and Pluto were classified as dwarf planets, a category distinct from the new definition of planet. Brown has since stated his approval of this classification. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris. == Orbit ==

, Uranus, Neptune, and Pluto (white/gray). The arcs below the ecliptic are plotted in darker colors, and the red dot is the Sun. The diagram on the left is a polar view whereas the diagrams on the right are different views from the ecliptic. . Eris has an orbital period of 561 years. The Eridian orbit is highly eccentric, and brings Eris to within 37.9 AU of the Sun, a typical perihelion for scattered objects. This is within the orbit of Pluto, but still safe from direct interaction with Neptune (~37 AU). Pluto, on the other hand, like other plutinos, follows a less inclined and less eccentric orbit and, protected by orbital resonance, can cross Neptune's orbit. In about 800 years, Eris will be closer to the Sun than Pluto for some time (see the graph at the left). As of 2007, Eris has an apparent magnitude of 18.7, making it bright enough to be detectable to some amateur telescopes. A telescope with a CCD can detect Eris under favorable conditions. The reason it had not been noticed until 2005 is its steep orbital inclination; searches for large outer Solar System objects tend to concentrate on the ecliptic plane, where most bodies are found. Because of the high inclination of its orbit, Eris passes through only a few constellations of the traditional Zodiac; it is in the constellation Cetus. It was in Sculptor from 1876 until 1929 and Phoenix from roughly 1840 until 1875. In 2036, it will enter Pisces and stay there until 2065, when it will enter Aries. It will then move into the northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in 2173. == Size, mass and density ==

{{Pie chart {"value":21.39, "label": "Triton $d"}, {"value":16.38, "label": "Eris $d"}, {"value":13.03, "label": "Pluto $d"}, {"value":4.01, "label": "Haumea $d"}, {"value":2.69, "label": "Makemake $d"}, {"value":1.75, "label": "Gonggong $d"}, {"value":1.59, "label": "Charon $d"}, {"value":1.21, "label": "Quaoar $d"}, ] }} In November 2010, Eris was the subject of one of the most distant stellar occultations yet from Earth. Tidal heating of Eris by its moon Dysnomia may additionally contribute to the preservation of its possible subsurface ocean. In July 2015, after nearly a decade of Eris being thought to be the ninth-largest known object to directly orbit the Sun, close-up imagery from the New Horizons mission determined the volume of Pluto to be slightly larger than that of Eris. Eris is thus the tenth-largest known object to directly orbit the Sun by volume and the ninth-largest by mass. == Surface and atmosphere ==

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. Infrared light from the object revealed the presence of methane ice, indicating that the surface may be similar to that of Pluto, which at the time was the only TNO known to have surface methane, and of Neptune's moon Triton, which also has methane on its surface. 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. Eris's comparatively low deuterium abundance suggests that its methane is not primordial and instead may have been produced from subsurface geochemical processes. Substantial quantities of nitrogen ice on Eris was also detected by the JWST, and it is presumed to have originated from subsurface processes similar to Eris's likely non-primordial methane. The abundance of nitrogen ice on Eris is estimated to be one-third of that of methane by volume. Unlike the somewhat reddish and variegated surfaces of Pluto and Triton, the surface of Eris appears almost white and uniform. Pluto's reddish color is thought to be due to deposits of tholins on its surface, and where these deposits darken the surface, the lower albedo leads to higher temperatures and the evaporation of methane deposits. In contrast, Eris is far enough from the Sun that methane can condense onto its surface even where the albedo is low. The condensation of methane uniformly over the surface reduces any albedo contrasts and would cover up any deposits of red tholins. This methane sublimation and condensation cycle could produce bladed terrain on Eris, similar to those on Pluto. Alternatively, Eris's surface could be refreshed through radiogenic convection of a global methane and nitrogen ice glacier, similar to Pluto's Sputnik Planitia. Spectroscopic observations by the JWST support the idea that Eris's surface is continually refreshing, as no signs of ethane, a byproduct of radiolyzed methane, were detected on Eris's surface. Due to the distant and eccentric orbit of Eris, its surface temperature is estimated to vary from about . 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. Because methane and nitrogen are both highly volatile, their presence shows either that Eris has always resided in the distant reaches of the Solar System, where it is cold enough for methane and nitrogen ice to persist, or that the celestial body has an internal source to replenish gas that escapes from its atmosphere. This contrasts with observations of another discovered TNO, , which reveal the presence of water ice but not methane. == Rotation ==

Eris displays very little variation in brightness as it rotates due to its uniform surface, making measurement of its rotation period difficult. Precise long-term monitoring of Eris's brightness indicates that it is tidally locked to its moon Dysnomia, with a rotation period synchronous with the moon's orbital period of 15.78 Earth days. 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. Previous measurements of Eris's rotation period obtained highly uncertain values ranging tens of hours to several days due to insufficient long-term coverage of Eris's rotation. The axial tilt of Eris has not been measured, 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. If this were the case, most of Eris's northern hemisphere would be illuminated by sunlight, with 30% of the hemisphere experiencing constant illumination in 2018. == Satellite ==

In 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris), using the newly commissioned laser guide star adaptive optics system. Images taken on September 10 revealed a moon in orbit around Eris. In keeping with the "Xena" nickname already in use for Eris, Brown's team nicknamed the moon "Gabrielle", after the television warrior princess's sidekick. When Eris received its official name from the IAU, the moon received the name Dysnomia, after the Greek goddess of lawlessness who was Eris's daughter. Brown says he picked it for similarity to his wife's name, Diane. The name also retains an oblique reference to Eris's old informal name Xena, portrayed on television by Lucy Lawless, though the connection was unintentional. == Exploration ==

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. It was calculated that a flyby mission to Eris would take 24.66 years using a Jupiter gravity-assist, based on launch dates of April 3, 2032, or April 7, 2044. Eris would be 92.03 or 90.19 AU from the Sun when the spacecraft arrives. == See also ==