# Eris (dwarf planet)

Last updated

Eris
Eris (center) and Dysnomia (left of center), taken by the Hubble Space Telescope
Discovery
Discovered by
Discovery dateJanuary 5, 2005 [2]
Designations
(136199) Eris
Pronunciation [3] or [4]
Named after
Ἔρις Eris
2003 UB313 [5]
Xena (nickname)
Orbital characteristics [5]
Epoch May 31, 2020
(JD 2459000.5)
Earliest precovery dateSeptember 3, 1954
Aphelion 97.457  AU (14.579  Tm)
Perihelion 38.271 AU (5.725 Tm)
67.864 AU (10.152 Tm)
Eccentricity 0.43607
559.07 yr (204,199 d)
Average orbital speed
3.434 km/s
205.989°
Inclination 44.040°
35.951°
151.639°
Known satellites Dysnomia
Mean diameter
2326±12 km
1163±6 km [10] [11]
(1.70±0.02)×107 km2 [lower-alpha 1]
Volume (6.59±0.10)×109 km3 [lower-alpha 1]
Mass
Mean density
2.52±0.07 g/cm3 [lower-alpha 2]
Equatorial surface gravity
0.82±0.02 m/s2
0.083±0.002 g [lower-alpha 3]
Equatorial escape velocity
1.38±0.01 km/s [lower-alpha 3]
Sidereal rotation period
25.92 hr [13]
28.03 hr [14]
78° (to orbit) [15]
0.96+0.09
−0.04
[ sic ] [10]
Surface temp. minmeanmax
(approx)30  K 42.5 K55 K
B−V=0.78, V−R=0.45 [16]
18.7 [17]
−1.17+0.06
−0.11
[lower-alpha 4]
34.4±1.4 milli-arcsec [19]

Eris (minor planet designation 136199 Eris) is the most massive [20] and second-largest known dwarf planet in the Solar System. Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its discovery was verified later that year. In September 2006 it was named after the goddess of strife and discord. Eris is the ninth-most massive object directly orbiting the Sun, and the sixteenth-most massive overall in the Solar System (including moons). It is also the largest object that has not been visited by a spacecraft. Eris has been measured at 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi) in diameter. [10] Its mass is 0.27 percent of the Earth's mass and 27 percent more than dwarf planet Pluto's, [12] [21] though Pluto is slightly larger by volume. [22]

## Contents

Eris is a trans-Neptunian object (TNO) and a member of a high-eccentricity population known as the scattered disk. It has one known moon, Dysnomia. In February 2016, its distance from the Sun was 96.3 astronomical units (1.441×1010 km; 8.95×109 mi), [17] roughly three times that of Pluto. With the exception of some long-period comets, until was discovered on December 17, 2018, Eris and Dysnomia were the most distant known natural objects in the Solar System. [17]

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 is a "dwarf planet," along with objects such as Pluto, Ceres, Haumea and Makemake, [23] thereby 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 its diameter was 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi), very slightly less than Pluto, [24] [25] which was measured by New Horizons as 2,376.6 ± 3.6 kilometers (1,476.8 ± 2.2 mi) in July 2015. [26] [27]

## Discovery

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. [28] The discovery was announced on July 29, 2005, the same day as Makemake and two days after Haumea, [29] 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 50000 Quaoar, 90482 Orcus, and 90377 Sedna. [30]

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. [28] 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 motion against the background stars. [28]

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. [28] 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. [5]

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 they calculated to be (1.66±0.02)×1022 kg, [12] 27%±2% greater than Pluto's.

## Name

Eris is named after the Greek goddess Eris (Greek Ἔρις), a personification of strife and discord. [31] The name was proposed by the Caltech team on September 6, 2006, and it was assigned on September 13, 2006, [32] 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.

Like the moons Io and Mimas, and for the same reason, the name Eris has two competing pronunciations, with a "long" and a "short" e. [33] The literary English pronunciation of the goddess is , with a long e. [3] However, Brown and his students [34] use something closer to Latin and Greek, , with a short e (ignoring the Great Vowel Shift that affects Classical names in English). [4]

The Greek and Latin oblique stem of the name is Erid-, [35] as can be seen in Italian Eride and Russian Эрида Erida, so the adjective in English is Eridian (). [8] [9]

### Xena

Due to uncertainty over whether the object would be classified as a planet or a minor planet, because different nomenclature procedures apply to these different classes of objects, [36] the decision on what to name the object had to wait until after the August 24, 2006 IAU ruling. [37] As a result, for a time the object became known to the wider public as Xena.

"Xena" was an informal name used internally by the discovery team. It was 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 (OK, so it's TV mythology, but Pluto is named after a cartoon, right?), [lower-alpha 5] and (this part is actually true) 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! [39]

"We assumed [that] a real name would come out fairly quickly, [but] the process got stalled", Mike Brown said in an interview:

One reporter [Ken Chang] [40] called me up from The New York Times who happened to have been a friend of mine from college, [and] I was a little less guarded with him than I am with the normal press. He 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. [41]

### Choosing an official 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. [30] The name was very similar to "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 for Sedna. [42]

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. [30]

Brown had also speculated that Persephone , the wife of the god Pluto, would be a good name for the object. [2] The name had been used several times in science fiction, [43] and was popular with the public, having handily won a poll conducted by New Scientist magazine ("Xena", despite only being a nickname, came fourth). [44] This was not possible once the object was classified as a dwarf planet, because there is already an asteroid with that name, 399 Persephone. [2]

With the dispute resolved, the discovery team proposed Eris on September 6, 2006. On September 13, 2006 this name was accepted as the official name by the IAU. [45] [46] Brown decided that, because the object had been considered a planet for so long, it deserved a name from Greek or Roman 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. [41] "Eris caused strife and discord by causing quarrels among people", said Brown in 2006, "and that's what this one has done too". [47]

## Classification

Eris is a trans-Neptunian dwarf planet (plutoid). [48] 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. [49]

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. [50] 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. [51] Brown has since stated his approval of this classification. [52] The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris. [37]

## Orbit

Eris has an orbital period of 559 years. [17] Its maximum possible distance from the Sun (aphelion) is 97.65  AU, and its closest (perihelion) is 37.91 AU. [17] It came to perihelion between 1698 [7] and 1699, [53] and to aphelion around 1977, [53] and will return to perihelion around 2256 [53] to 2258. [54] Unlike the eight planets, whose orbits all lie roughly in the same plane as the Earth's, Eris's orbit is highly inclined: It is tilted at an angle of about 44  degrees to the ecliptic. [5] When discovered, Eris and its moon were the most distant known objects in the Solar System, apart from long-period comets and space probes. [2] [55] It retained this distinction until the discovery of in 2018. [56]

As of 2008 there were approximately forty known TNOs, most notably , and Sedna, that are currently closer to the Sun than Eris even though their semimajor axis is larger than that of Eris (67.8 AU). [6]

Eris's orbit is highly eccentric, and brings Eris to within 37.9 AU of the Sun, a typical perihelion for scattered objects. [57] This is within the orbit of Pluto, but still safe from direct interaction with Neptune (~37 AU). [58] 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. [59] 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. [60] A 200-millimeter (7.9 in) telescope with a CCD can detect Eris under favorable conditions. [lower-alpha 6] 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. [61]

Because of the high inclination of its orbit, Eris passes through only a few constellations of the traditional Zodiac; it is now 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. [53] 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

Size estimates
20051,199 (2,397) km [62] Hubble
20071,300 (2,600) km [63] Spitzer
20111,163 (2,326) km [10] Occultation

In November 2010, Eris was the subject of one of the most distant stellar occultations yet from Earth. [11] Preliminary data from this event cast doubt on previous size estimates. [11] The teams announced their final results from the occultation in October 2011, with an estimated diameter of 2326±12 km. [10]

This makes Eris a little smaller than Pluto by area and diameter, which is 2372±4 km across, although Eris is more massive. It also indicates an albedo of 0.96, higher than that of any other large body in the Solar System except Enceladus. [10] 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. [19]

The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia's period—15.774 days [12] [64] —Eris is 27 percent more massive than Pluto. Using the 2011 occultation results, Eris has a density of 2.52±0.07 g/cm3, [lower-alpha 2] substantially denser than Pluto, and thus must be composed largely of rocky materials. [10]

Models of internal heating via radioactive decay suggest that Eris could have an internal ocean of liquid water at the mantle–core boundary. [65]

In July 2015, after nearly ten years of Eris being considered the ninth-largest object known to directly orbit the sun, close-up imagery from the New Horizons mission more accurately determined Pluto's volume to be slightly larger than Eris's, rather than slightly smaller as previously thought. [66] Eris is now the tenth-largest object known to directly orbit the sun by volume, but remains 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. [67]

Due to Eris's distant eccentric orbit, its surface temperature is estimated to vary between about 30 and 56 K (−243.2 and −217.2 °C). [2]

Unlike the somewhat reddish Pluto and Triton, Eris appears almost white. [2] 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. [28]

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. Because methane is highly volatile, its presence shows either that Eris has always resided in the distant reaches of the Solar System, where it is cold enough for methane ice to persist, or that the celestial body has an internal source of methane to replenish gas that escapes from its atmosphere. This contrasts with observations of another discovered TNO, Haumea, which reveal the presence of water ice but not methane. [68]

## 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. [70] 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' 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. [71]

## Exploration

In the 2010s, on the heels of the successful Pluto flyby there were multiple studies for follow-on missions to explore the Kuiper belt, and Eris was evaluated among the candidates. [72] It was calculated that a flyby mission to Eris could 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. [73]

## Notes

1. Calculated from the mean radius
2. Calculated by dividing the listed mass by the listed volume
3. Calculated based on the known parameters
4. Calculated from the listed diameter (D) and albedo (p) using ${\displaystyle H=-5\log {D{\sqrt {p}} \over 1329}}$ [18]
5. Brown is joking at this point. It was in fact the Disney character Pluto that was named after the newly discovered planet, though Venetia Phair, Pluto's christener, had to counter accusations her whole life that she named the planet after a cartoon dog. [38]
6. For an example of an amateur image of Eris, see Fred Bruenjes' Astronomy Archived November 2, 2005, at the Wayback Machine

## Related Research Articles

The Kuiper belt, occasionally called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger – 20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet or dwarf planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (AU).

Pluto is an icy dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It was the first and the largest Kuiper belt object to be discovered.

90482 Orcus, provisional designation 2004 DW, is a trans-Neptunian object with a large moon, Vanth. With a diameter of 910 km (570 mi), it is a possible dwarf planet. The surface of Orcus is relatively bright with albedo reaching 23 percent, neutral in color and rich in water ice. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane or ammonia may also be present on its surface. It was discovered by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz on 17 February 2004.

Chadwick A. Trujillo is an American astronomer, discoverer of minor planets and the co-discoverer of Eris, the most massive dwarf planet known in the Solar System.

Michael E. Brown is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. His team has discovered many trans-Neptunian objects (TNOs), indlucing the dwarf planet Eris, which was originally thought to be bigger than Pluto, triggering a debate on the definition of a planet.

The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai, "wandering stars", for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different objects, from the Sun and the Moon to satellites and asteroids.

Haumea is a likely dwarf planet located beyond Neptune's orbit. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory in the United States and independently in 2005 by a team headed by José Luis Ortiz Moreno at the Sierra Nevada Observatory in Spain, though the latter claim has been contested. On September 17, 2008, it was named after Haumea, the Hawaiian goddess of childbirth, under the expectation by the International Astronomical Union (IAU) that it would prove to be a dwarf planet. It is probably the third-largest known trans-Neptunian object, after Eris and Pluto.

Makemake is a likely dwarf planet and perhaps the second largest Kuiper belt object in the classical population, with a diameter approximately two-thirds that of Pluto. Makemake has one known satellite, S/2015 (136472) 1. Makemake's extremely low average temperature, about 40 K (−230 °C), means its surface is covered with methane, ethane, and possibly nitrogen ices.

Dysnomia (formally (136199) Eris I Dysnomia) is the only known moon of the dwarf planet Eris and likely the second-largest known moon of a dwarf planet, after Pluto I Charon. It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M. Keck Observatory, and carried the provisional designation of S/2005 (2003 UB313) 1 until officially named Dysnomia (from the Ancient Greek word Δυσνομία meaning anarchy/lawlessness) after the daughter of the Greek goddess Eris.

A dwarf planet is a planetary-mass object that does not dominate its region of space and is not a satellite. That is, it is in direct orbit of the Sun and is massive enough to be plastic – for its gravity to maintain it in a hydrostatically equilibrious shape – but has not cleared the neighborhood around its orbit of other material. The prototype dwarf planet is Pluto.

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body which:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium, and
3. has "cleared the neighborhood" around its orbit.

The outer Solar System dwarf planet Haumea has two known moons, Hiʻiaka and Namaka, named after Hawaiian goddesses. These small moons were discovered in 2005, from observations of Haumea made at the large telescopes of the W. M. Keck Observatory in Hawaii.

Haumea was the first of the IAU-recognized dwarf planets to be discovered since Pluto in 1930. Its naming as a dwarf planet was delayed by several years due to controversy over who should receive credit for its discovery. A California Institute of Technology (Caltech) team headed by Michael E. Brown first noticed the object, but a Spanish team headed by José Luis Ortiz Moreno were the first to announce it, and so normally would receive credit. Brown accused the Spanish team of fraud, using Caltech observations without credit to make their discovery, while the Ortiz team accused the American team of political interference with the International Astronomical Union (IAU). The IAU officially recognized the Californian team's proposed name Haumea over the name proposed by the Spanish team, Ataecina, in September 2008.

225088 Gonggong (minor planet designation: 2007 OR10) is a likely dwarf planet of the Solar System, and a member of the scattered disc beyond Neptune. It has a highly eccentric and inclined orbit during which it ranges from 34–101 astronomical units (5.1–15.1 billion kilometers; 3.2–9.4 billion miles) from the Sun. As of 2019, its distance from the Sun is 88 AU (13.2×109 km; 8.2×109 mi), and it is the sixth-farthest known Solar System object. Gonggong is in a 3:10 orbital resonance with Neptune, in which it completes three orbits around the Sun for every ten orbits completed by Neptune. Gonggong was discovered in July 2007 by American astronomers Megan Schwamb, Michael Brown, and David Rabinowitz at the Palomar Observatory, and the discovery was announced in January 2009.

50000 Quaoar is a non-resonant trans-Neptunian object (cubewano) and a possible dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. It measures approximately 1,121 km (697 mi) in diameter, about half the diameter of Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory on 4 June 2002. Signs of water ice on the surface of Quaoar have been found, which suggests that cryovolcanism may be occurring on Quaoar. A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects. In February 2007, Weywot, a synchronous moon in orbit around Quaoar, was discovered by Brown. Weywot is measured to be 170 km (110 mi) across. Both objects were named after mythological figures from the Native American Tongva people in Southern California. Quaoar is the Tongva creator deity and Weywot is his son.

The region of the Solar System beyond Neptune contains sparse populations of relatively small rocky and icy objects. These include the Kuiper belt, a group which includes the dwarf planet Pluto and other plutoids including Haumea and Makemake. Further out is the scattered disk, a group which includes Eris, a dwarf planet slightly smaller than Pluto, and even more distant detached objects such as Sedna.

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