This article is about the dwarf planet. For the deity, seeEris (mythology). For the asteroid with a similar name, see433 Eros. For other uses, seeEris.
Eris has one large knownmoon,Dysnomia. In February 2016, Eris's distance from the Sun was 96.3 AU (14.41 billion km; 8.95 billion mi),[20] more than three times that ofNeptune or Pluto. With the exception oflong-period comets, Eris and Dysnomia were the most distant known natural objects in the Solar System until the discovery of2018 AG37 and2018 VG18 in 2018.[20]
Because Eris appeared to be larger than Pluto,NASA initially described it as the Solar System'stenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated theInternational Astronomical Union (IAU) todefine the termplanet for the first time. Under theIAU definition approved on August 24, 2006, Eris, Pluto andCeres are "dwarf planets",[26] reducing the number of known planets in the Solar System to eight, the same as before Pluto's discovery in 1930. Observations of a stellaroccultation by Eris in 2010 showed that it was slightly smaller than Pluto,[27][28] which was measured byNew Horizons as having amean diameter of 2,377 ± 4 kilometres (1,477 ± 2 mi) in July 2015.[29][30]
Routine observations were taken by the team on October 21, 2003, using the 1.2 mSamuel OschinSchmidt telescope atPalomar 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 offalse 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. In January 2005, the re-analysis revealed Eris's slow orbital motion against the backgroundstars.[31]
Follow-up observations were then carried out to make a preliminary determination of Eris'sorbit, which allowed the object's distance to be estimated.[31] 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.[2]
Precovery images of Eris have been identified back to September 3, 1954.[6]
More observations released in October 2005 revealed that Eris has a moon, later namedDysnomia. 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.
Eris is named after theGreek goddessEris (GreekἜρις), a personification of strife and discord.[34] The name was proposed by the Caltech team on September 6, 2006, and it was assigned on September 13, 2006,[35] following an unusually long period in which the object was known by theprovisional designation2003 UB313, which was granted automatically by the IAU under their naming protocols forminor planets.
The nameEris has two competing pronunciations, with a "long" or with a "short"e, analogous to the two competing pronunciations of the wordera.[4] Perhaps the more common form in English, used i.a. by Brown and his students,[36] is/ˈɛrɪs/ withdisyllabic laxing and a shorte.[3] However, the classical English pronunciation of the goddess is/ˈɪərɪs/, with a longe.[5]
The Greek and Latinoblique stem of the name isErid-,[37] as can be seen in ItalianEride and Russian ЭридаErida, so the adjective in English isEridian/ɛˈrɪdiən/.[9][10]
Due to uncertainty over whether the object would be classified as aplanet or aminor planet, because varying nomenclature procedures apply to these classes of objects,[38] the decision on what to name the object had to wait until after the August 24, 2006, IAU ruling.[39] For a period of time, the object became known to the wider public asXena. "Xena" was an informal name used internally by the discovery team, inspired by the title character of the television seriesXena: 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![40]
Brown said in an interview that the naming process was stalled:
One reporter [Ken Chang][41] called me up fromThe 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.[42]
Animation showing the movement of Eris on the images used to discover it. Eris is indicated by the arrow. The three frames were taken over a period of three hours.
According to science writerGovert Schilling, Brown initially wanted to call the object "Lila", after a concept inHindu mythology that described the cosmos as the outcome of a game played byBrahman.[33] 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 withSedna 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.[43]
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.[33]
Brown had also consideredPersephone, the wife of the godPluto, as a name for the object.[2] The name had been used several times for planets inscience fiction[44] and was popular with the public, having handily won a poll conducted byNew Scientist magazine.[45] ("Xena", despite only being a nickname, came fourth.) This choice was not possible because there was already aminor planet with that name,399 Persephone, and Eris was being named as a minor planet.[2]
The discovery team proposedEris on September 6, 2006. Brown decided that, because the object had been considered aplanet for so long, it deserved a name fromGreek orRoman mythology like the other planets. The asteroids had taken the vast majority of Graeco-Roman names.Eris, whom Brown described as his favorite goddess, had fortunately escaped inclusion.[42] "Eris caused strife and discord by causing quarrels among people," said Brown in 2006, "and that's what this one has done too."[46] The name was accepted by the IAU on September 13, 2006.[47][48]
Although the usage ofplanetary symbols is generally discouraged in astronomy, NASA has used theHand of Eris,⟨⟩ (U+2BF0), for Eris.[49] This symbol was taken fromDiscordianism, a religion concerned with the goddess Eris.[50] TheSternberg Astronomical Institute atMoscow State University has used (U+24C0), presumably either the "all rights reversed" symbol of thePrincipia Discordia or a simplification of theApple of Discord inscribed with the Greek wordKallisti,, which had been suggested as a symbol for the dwarf planet on the Discordian discussion board that eventually settled on⟨⟩.[51][52]Most astrologers use the Hand of Eris, though other symbols are occasionally seen, such as⟨⟩ (U+2BF1).[50]
Eris is a trans-Neptunian dwarf planet. Its orbital characteristics more specifically categorize it as ascattered-disk object (SDO), or a TNO that has been "scattered" from theKuiper belt into more-distant and unusualorbits following gravitational interactions withNeptune as theSolar 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.[53]
Because Eris was initially thought to be larger thanPluto, it was described as the "tenth planet" byNASA and in media reports of its discovery.[54] In response to the uncertainty over its status, and because of ongoing debate over whether Pluto should be classified as aplanet, the IAU delegated a group of astronomers to develop a sufficiently precise definition of the termplanet to decide the issue. This was announced as the IAU'sDefinition of a Planet in the Solar System, adopted on August 24, 2006. At this time, both Eris and Pluto were classified asdwarf planets, a category distinct from the new definition ofplanet.[55] Brown has since stated his approval of this classification.[56] The IAU subsequently added Eris to itsMinor Planet Catalogue, designating it(136199) Eris.[39]
The orbit of Eris (blue) compared to those ofSaturn,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.Seen from Earth, Eris makes small loops in the sky through the constellation ofCetus.
Eris has anorbital period of 561years.[6] Its maximum possible distance from theSun (aphelion) is 97.7 AU, and its closest (perihelion) is 38.4 AU.[6] As the time of perihelion is defined at theepoch chosen using an unperturbedtwo-body solution, the further the epoch is from the date of perihelion, the less accurate the result.Numerical integration is required to predict the time of perihelion accurately. Numerical integration byJPL Horizons shows that Eris came to perihelion around 1699,[57] to aphelion around 1977, and will return to perihelion around December 2257.[11] Unlike those of the eight planets, whose orbits all lie roughly in the same plane as the Earth's, Eris's orbit is highlyinclined: it is tilted at an angle of about 44 degrees to theecliptic.[6] When discovered, Eris and its moon were the most distant known objects in the Solar System, apart fromlong-period comets andspace probes.[2][58] It retained this distinction until the discovery of2018 VG18 in 2018.[59]
As of 2008, there were approximately forty knownTNOs, most notably2006 SQ372,2000 OO67 andSedna, that are currently closer to the Sun than Eris, even though theirsemimajor axis is larger than that of Eris (67.8 AU).[7]
The distances of Eris and Pluto from the Sun in the next 1,000 years
The Eridian orbit is highlyeccentric, and brings Eris to within 37.9 AU of the Sun, a typical perihelion forscattered objects.[60] This is within the orbit of Pluto, but still safe from direct interaction with Neptune (~37 AU).[61] Pluto, on the other hand, like otherplutinos, follows a less inclined and less eccentric orbit and, protected byorbital resonance, can cross Neptune's orbit.[62] 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 anapparent magnitude of 18.7, making it bright enough to be detectable to some amateurtelescopes.[63] A 200-millimetre (7.9 in) telescope with aCCD can detect Eris under favorable conditions.[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.[64]
Because of the high inclination of its orbit, Eris passes through only a few constellations of the traditionalZodiac; it is now in the constellationCetus. It was inSculptor from 1876 until 1929 andPhoenix from roughly 1840 until 1875. In 2036, it will enterPisces and stay there until 2065, when it will enterAries.[65] It will then move into thenorthern sky, enteringPerseus in 2128 andCamelopardalis (where it will reach its northernmostdeclination) in 2173.
Size comparison: Eris (lower left) with the Moon and Earth (top and right)
Illustration of the stellar occultation by Eris in November 2010. The resulting occultationchords project a circular silhouette for Eris, giving a spherical diameter of 2,326 km (1,445 mi)
Graphs are unavailable due to technical issues. Updates on reimplementing the Graph extension, which will be known as the Chart extension, can be found onPhabricator and onMediaWiki.org.
In November 2010, Eris was the subject of one of the most distantstellar occultations yet from Earth.[13] Preliminary data from this event cast doubt on previous size estimates.[13] The teams announced their final results from the occultation in October 2011, with an estimated diameter of2326±12 km.[12]
This makes Eris a little smaller than Pluto by area and diameter, which is2372±4 km across, although Eris is more massive. It also indicates ageometric albedo of 0.96. 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.[21]
The mass of Eris can be calculated with much greater precision. Based on the accepted value for Dysnomia's period at the time—15.774 days[23][68]—Eris is 27% more massive than Pluto. Using the 2011 occultation results, Eris has a density of2.52±0.07 g/cm3,[g] substantially denser than Pluto, and thus must be composed largely of rocky materials.[12]
Models of internal heating viaradioactive decay suggest that Eris could have asubsurface ocean of liquid water at the mantle–core boundary.[69]Tidal heating of Eris by its moon Dysnomia may additionally contribute to the preservation of its possible subsurface ocean.[70] More research concluded that Eris, Pluto and Makemake could harbor active subsurface oceans and show active geothermal activity.[71]
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 theNew Horizons mission determined the volume of Pluto to be slightly larger than that of Eris.[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 infrared spectrum of Eris, compared to that of Pluto, shows the marked similarities between the two bodies. Arrows denote methane absorption lines.
The discovery team followed up their initial identification of Eris withspectroscopic observations made at the 8 mGemini North Telescope in Hawaii on January 25, 2005. Infrared light from the object revealed the presence ofmethane ice, indicating that the surface may be similar to that of Pluto, which at the time was the onlyTNO known to have surface methane, and of Neptune's moonTriton, which also has methane on its surface.[73] In 2022, near-infrared spectroscopy of Eris by theJames Webb Space Telescope (JWST) revealed the presence ofdeuterated methane ice on its surface, at abundances lower than those inJupiter-family comets like67P/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.[74] Substantial quantities ofnitrogen 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.[74] The abundance of nitrogen ice on Eris is estimated to be one-third of that of methane by volume.[74]
Unlike the somewhat reddish and variegated surfaces of Pluto and Triton, the surface of Eris appears almost white and uniform.[2] Pluto's reddish color is thought to be due to deposits oftholins 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 cancondense 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.[31] This methane sublimation and condensation cycle could producebladed terrain on Eris, similar to those on Pluto.[75][74] Alternatively, Eris's surface could be refreshed throughradiogenicconvection of a global methane and nitrogen ice glacier, similar to Pluto'sSputnik Planitia.[76][74] Spectroscopic observations by the JWST support the idea that Eris's surface is continually refreshing, as no signs ofethane, a byproduct ofradiolyzed methane, were detected on Eris's surface.[74]
Due to the distant and eccentric orbit of Eris, its surface temperature is estimated to vary from about 30 to 56 K (−243.2 to −217.2 °C; −405.7 to −358.9 °F).[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 tosublime to form anatmosphere. Because methane and nitrogen are both highlyvolatile, 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.[75] This contrasts with observations of another discovered TNO,Haumea, which reveal the presence ofwater ice but not methane.[77]
Eris displays very little variation in brightness as it rotates due to its uniform surface, making measurement of itsrotation period difficult.[78][15] Precise long-term monitoring of Eris's brightness indicates that it istidally locked to its moon Dysnomia, with a rotation periodsynchronous with the moon's orbital period of 15.78Earth days.[15] Dysnomia is also tidally locked to Eris, which makes the Eris–Dysnomia system the second known case of double-synchronous rotation, after Pluto andCharon. 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.[78][79][80] Theaxial 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.[16] 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.[16]
Artist's conception of Eris and its dark moon Dysnomia
In 2005, theadaptive optics team at theKeck telescopes in Hawaii carried out observations of the four brightestTNOs (Pluto, Makemake, Haumea, and Eris), using the newly commissionedlaser guide star adaptive optics system.[81] Images taken on September 10 revealed amoon 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 nameDysnomia, after theGreek 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 nameXena, portrayed on television byLucy Lawless, though the connection was unintentional.[82]
Eris was observed from afar by the outboundNew Horizons spacecraft in May 2020, as part of its extended mission following its successful Plutoflyby in 2015.[17] Although Eris was farther fromNew 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 highphase angles that are otherwise unobtainable from Earth, enabling the determination of the light scattering properties andphase curve behavior of the Eridian surface.[17]
In the 2010s, there were multiple studies for follow-on missions to explore the Kuiper belt, among which Eris was evaluated as a candidate.[84] 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.[85]
^abThe mass of Eris by itself is the difference between the mass of the system ((1.6466±0.0085)×1022 kg)[14] and mass of Dysnomia by itself ((8.2±5.7)×1019 kg).[83]
^abAssumed axial tilt if Eris rotates in the same plane as Dysnomia's orbit, which is tilted 78.29° with respect to Eris's orbit.[14]
^Holler et al. (2021) determined anecliptic latitude ofβ = 28.41° for the north pole of Dysnomia's orbit, which is assumed to be similarly oriented as Eris's rotational north pole.[14][16]β is the angular offset from theecliptic plane, whereasinclinationi with respect to the ecliptic is the angular offset from theecliptic north pole atβ = +90° ;i with respect to the ecliptic would be thecomplement ofβ. Therefore, givenβ = 28.41° ,i = 90° – (28.41°) = 61.59° from the ecliptic.
^abcdHoller, Bryan J.; Grundy, William; Buie, Marc W.; Noll, Keith (October 2018).Breaking the degeneracy of Eris' pole orientation. 50th DPS Meeting. American Astronomical Society.Bibcode:2018DPS....5050903H. 509.03.
^Thomas H. Maugh II; John Johnson Jr. (October 16, 2005)."His Stellar Discovery Is Eclipsed".Los Angeles Times.Archived from the original on October 12, 2012. RetrievedJuly 14, 2008.
^Blue, Jennifer (September 14, 2006)."2003 UB 313 named Eris".USGS Astrogeology Research Program. Archived fromthe original on October 18, 2006. RetrievedJanuary 3, 2007.
^Brown, Mike."New Planet".web.gps.caltech.edu.Archived from the original on May 17, 2012. RetrievedJanuary 5, 2010.
^"M.P.C. 52733"(PDF).Minor Planet Circulars. Minor Planet Center. 2004. p. 1.Archived(PDF) from the original on July 25, 2011. RetrievedAugust 30, 2010.
^JPL/NASA (April 22, 2015)."What is a Dwarf Planet?".Jet Propulsion Laboratory.Archived from the original on December 8, 2021. RetrievedSeptember 24, 2021.
^John Stansberry; Will Grundy; Mike Brown; John Spencer; David Trilling; Dale Cruikshank; Jean-Luc Margot (2007).Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope.arXiv:astro-ph/0702538.Bibcode:2008ssbn.book..161S.
^Grundy, William M.; Umurhan, Orkan M. (October 2017).Are Makemake and Eris Sputnik Planets?. 49th DPS Meeting. American Astronomical Society.Bibcode:2017DPS....4920202G. 202.02.
^McGranaghan, R.; Sagan, B.; Dove, G.; Tullos, A.; Lyne, J. E.; Emery, J. P. (2011). "A Survey of Mission Opportunities to Trans-Neptunian Objects".Journal of the British Interplanetary Society.64:296–303.Bibcode:2011JBIS...64..296M.
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