Eris (dwarf planet)

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This article is about the dwarf planet. For the asteroid, see 433 Eros.

Eris
Eris (center) and Dysnomia (left of center), taken by the Hubble Space Telescope
Discovery
Discovered by
Discovery dateJanuary 5, 2005[2][a]
Designations
Designation
(136199) Eris
Pronunciation/ˈɪərɪs/ or /ˈɛrɪs/[b]
Named after
Eris
2003 UB313[5]
AdjectivesEridian
Orbital characteristics[5]
Epoch December 9, 2014
(JD 2457000.5)
Aphelion
  • 97.651 AU
  • 14.602×109 km
Perihelion
  • 37.911 AU
  • 5.723×109 km
  • 67.781 AU
  • 10.166×109 km
Eccentricity0.44068
  • 203,830 d
  • 558.04 yr
3.4338 km/s
204.16°
Inclination44.0445°
35.9531°
150.977°
Known satellitesDysnomia
Physical characteristics
Mean radius
1163±6 km[8][9]
(1.70±0.02)×107 km2[c]
Volume(6.59±0.10)×109 km3[c]
Mass
Mean density
2.52±0.07 g/cm3[d]
0.82±0.02 m/s2
0.083±0.002 g[e]
1.38±0.01 km/s[e]
25.9±0.5 hr[11]
Albedo0.96+0.09
−0.04[8]
Surface temp. min mean max
(approx) 30 K 42.5 K 55 K
Spectral type
B−V=0.78, V−R=0.45[12]
18.7[13]
−1.17+0.06
−0.11[f]
40 milli-arcsec[15]

Eris (minor-planet designation 136199 Eris) is the most massive and second-largest dwarf planet known in the Solar System. Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. In September 2006 it was named after Eris, the Greek goddess of strife and discord.

Among known bodies of the Solar System, Eris is the ninth most massive directly orbiting the Sun, and the 16th most massive overall, because seven moons are more massive than all known dwarf planets. It is also the largest which has not yet been visited by a spacecraft. Eris was measured to be 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi) in diameter.[8] Eris' mass is about 0.27% of the Earth mass,[10][16] about 27% more than dwarf planet Pluto, although Pluto is slightly larger by volume.[17]

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. As of February 2016, its distance from the Sun was 96.3 astronomical units (1.441×1010 km; 8.95×109 mi),[13] roughly three times that of Pluto. With the exception of some long-period comets, Eris and Dysnomia are currently the second-most-distant natural objects in the Solar System,[18][g] the farthest object being V774104 discovered in November 2015 at 103 AU.[21]

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,[22] 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,[23][24] which was measured by New Horizons as 2372±4 km in July 2015.[25]

History

Discovery

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.

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. The discovery was announced on July 29, 2005, the same day as Makemake and two days after Haumea,[26] 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.

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, 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.

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.[2]

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,[10] 27%±2% greater than Pluto's.

Name

Athenian painting of Eris, c. 550 BC

Eris is named after the Greek goddess Eris (Greek Ἔρις), a personification of strife and discord.[27] The name was assigned on September 13, 2006, 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. The regular adjectival form of Eris is Eridian.

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,[28] the decision on what to name the object had to wait until after the August 24, 2006, IAU ruling.[29] 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?),[h] 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![31]

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

One reporter [Ken Chang][32] 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.[33]

Choosing an official name

Artist's impression of the dwarf planet Eris. This artistic representation is based on observations made at ESO's La Silla Observatory.[34]

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 Brahma. 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.[35]

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.[36]

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,[37] and was popular with the public, having handily won a poll conducted by New Scientist magazine ("Xena", despite only being a nickname, came fourth).[38] 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.[39][40] 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.[33] The name in part reflects the discord in the astronomical community caused by the debate over the object's (and Pluto's) classification.

Classification

Distribution of trans-Neptunian objects

Eris is a trans-Neptunian dwarf planet (plutoid).[41] 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.[42] Inner-belt objects are expected to be generally more massive than outer-belt objects, and so astronomers expect to discover more large objects like Eris in high-inclination orbits, which have traditionally been neglected.

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.[43] 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.[44] Brown has since stated his approval of this classification.[45] The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.[29]

Orbit

The orbit of Eris (blue) compared to those of Saturn, 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 558 years.[13] Its maximum possible distance from the Sun (aphelion) is 97.65 AU, and its closest (perihelion) is 37.91 AU.[13] It came to perihelion between 1698[7] and 1699,[46] to aphelion around 1977,[46] and will return to perihelion around 2256[46] to 2258.[11] Eris and its moon are currently the second most distant known objects in the Solar System, after V774104,[21] apart from long-period comets and space probes.[2][47] Approximately forty known TNOs, most notably 2006 SQ372, 2000 OO67 and Sedna, though currently closer to the Sun than Eris, have greater average orbital distances than Eris's semimajor axis of 67.7 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. This is within the orbit of Pluto, but still safe from direct interaction with Neptune (29.8–30.4 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. It is possible that Eris is in a 17:5 resonance with Neptune, though further observations will be required to check that hypothesis.[48] 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. In about 800 years, Eris will be closer to the Sun than Pluto for some time (see the graph at the right).

The distances of Eris and Pluto from the Sun in the next 1,000 years

As of February 2016, Eris has an apparent magnitude of 18.7, making it bright enough to be detectable to some amateur telescopes. A 200-millimeter (7.9 in) telescope with a CCD can detect Eris under favorable conditions.[i] 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.

Because of the high inclination of its orbit, Eris only passes through 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.[46] It will then move into the northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in 2173.

Positions of known outer Solar System objects
  Sun
  Jupiter trojans (6,178)
  Scattered disc (>300)   Giant planets: J · S · U · N
  Centaurs (44,000)
  Kuiper belt (>1,000)
(scale in AU; epoch as of January 2015; # of objects in parentheses)

These Solar System minor planets are the furthest from the Sun as of December 2021. The objects have been categorized by their approximate current distance from the Sun, and not by the calculated aphelion of their orbit. The list changes over time because the objects are moving in their orbits. Some objects are inbound and some are outbound. It would be difficult to detect long-distance comets if it were not for their comas, which become visible when heated by the Sun. Distances are measured in astronomical units (AU, Sun–Earth distances). The distances are not the minimum (perihelion) or the maximum (aphelion) that may be achieved by these objects in the future.

This list does not include near-parabolic comets of which many are known to be currently more than 100 AU (15 billion km) from the Sun, but are currently too far away to be observed by telescope. Trans-Neptunian objects are typically announced publicly months or years after their discovery, so as to make sure the orbit is correct before announcing it. Due to their greater distance from the Sun and slow movement across the sky, trans-Neptunian objects with observation arcs less than several years often have poorly constrained orbits. Particularly distant objects take several years of observations to establish a crude orbit solution before being announced. For instance, the most distant known trans-Neptunian object 2018 AG37 was discovered by Scott Sheppard in January 2018 but was announced three years later in February 2021.[49]

Noted objects

One particularly distant body is 90377 Sedna, which was discovered in November 2003. It has an extremely eccentric orbit that takes it to an aphelion of 937 AU.[50] It takes over 10,000 years to orbit, and during the next 50 years it will slowly move closer to the Sun as it comes to perihelion at a distance of 76 AU from the Sun.[51] Sedna is the largest known sednoid, a class of objects that play an important role in the Planet Nine hypothesis.

Pluto (30–49 AU, about 34 AU in 2015) was the first Kuiper belt object to be discovered (1930) and is the largest known dwarf planet.

Gallery

Known distant objects

This is a list of known objects at heliocentric distances of more than 65 AU. In theory, the Oort cloud could extend over 120,000 AU (2 ly) from the Sun.

Most distant observable objects in the Solar System as of December 2021[18]
Object name Distance from the Sun (AU) Radial velocity
(AU/yr)[j] 		Perihelion Aphelion Semimajor
axis 		Apparent
magnitude 		Absolute
magnitude (H) 		Important dates
December 2021 December 2015 Discovered Announced
Great Comet of 1680
(for comparison) 		258.0[52] 255.4[52] +0.47[52] 0.006 889 444 Unknown Unknown 1680-11-14 —
Voyager 1
(for comparison) 		152.9[52] 133.3[52] +3.57[52] 8.90 ∞
Hyperbolic		 −3.2[53] ~50 ~28 — —
2018 AG37 132.9±1.8 131.9±10.7 ±0.2(?) 27.1 145.0 86.0 25.4 4.2 2018-01-15 2021-02-10
Voyager 2
(for comparison) 		129.4[52] 109.7[52] +3.17[52] 21.2 ∞
Hyperbolic		 −4.0[53] ~48 ~28 — —
Pioneer 10
(for comparison) 		128.9[52] 114.8[52] +2.51[52] 4.94 ∞
Hyperbolic		 ~49 ~29 — —
2018 VG18 123.6 123.2 +0.06 37.8 123.9 81.3 24.6 3.7 2018-11-10 2018-12-17
2020 BE102 110.9 111.7 32.9 116.9 74.9 25.6 5.1 2020-01-24 2022-05-31
Pioneer 11
(for comparison) 		107.7[52] 92.5[52] +2.35[52] 9.45 ∞
Hyperbolic		 ~48 ~29 — —
2020 FY30 98.9 99.9 –0.17 35.6 107.7 71.6 24.8 4.7 2020-03-24 2021-02-14
2020 FA31 97.3 96.5 +0.14 39.5 102.4 71.0 25.4 5.4 2020-03-24 2021-02-14
Eris
136199 		95.9 96.3 −0.07 38.3 97.5 67.9 18.8 −1.21 2003-10-21 2005-07-29
2020 FQ40 92.4 92.7 –0.05 38.2 93.1 65.6 25.7 6.1 2020-03-24 2022-05-31
2015 TH367[k] 90.3 88.2 +0.42 28.9 136.4 82.6 26.3 6.6 2015-10-13 2018-03-13
2021 DR15 89.6 88.6 +0.17(?) 37.8 96.5 67.2 23.1 3.6 2021-02-17 2021-12-17
2014 UZ224 89.5 92.0 −0.45 38.3 177.0 107.6 23.2 3.4 2014-10-21 2016-08-28
Gonggong
225088 		88.7 87.4 +0.23 33.7 101.2 67.5 21.5 1.6 2007-07-17 2009-01-07
2015 FG415 87.2 87.9 −0.14 36.2 92.1 64.1 25.5 6.0 2015-03-17 2019-03-27
2014 FC69 85.5 84.1 +0.26 40.4 104.4 72.4 24.2 4.6 2014-03-25 2015-02-11
2006 QH181 84.6 83.3 +0.22 37.5 96.7 67.1 23.7 4.3 2006-08-21 2006-11-05
Sedna
90377 		84.2 85.8 −0.29 76.3 892.6 484.4 21.0 1.3 2003-11-14 2004-03-15
2015 VO166 84.3 82.5 +0.32 38.3 113.2 75.8 25.5 5.9 2015-11-06 2018-10-02
2012 VP113 84.2 83.3 +0.16 80.4 442.6 261.5 23.5 4.0 2012-11-05 2014-03-26
2013 FS28 83.5 85.9 −0.62 34.2 358.2 196.2 24.3 4.9 2013-03-16 2016-08-29
2017 SN132 82.8 80.4 +0.44 42.0 110.0 76.0 25.2 5.8 2017-09-16 2019-02-10
2019 EU5 81.7 85.5 46.5 2310 1178 25.6 6.4 2019-03-05 2021-12-17
2015 UH87[k] 81.3 82.3 −0.19 34.3 90.0 62.2 25.2 6.0 2015-10-16 2018-03-12
2013 FY27
532037 		79.7 80.3 −0.10 35.2 82.1 58.7 22.2 3.2 2013-03-17 2014-03-31
2021 DP15 79.7 76.2 29.1 204.1 116.6 25.4 6.2 2021-02-16 2021-12-17
2015 TJ367[k] 79.4 77.1 +0.42 33.6 128.1 80.9 25.8 6.7 2015-10-13 2018-03-13
2017 FO161 78.1 79.1 −0.18 34.1 85.5 59.8 23.3 4.3 2017-03-23 2018-04-02
Leleākūhonua
541132 		77.6 79.8 −0.40 65.2 2,106 1,085 24.6 5.5 2015-10-13 2018-10-01
2018 AD39 77.2 74.1 –0.58 38.4 287.9 163.2 25.0 6.2 2018-01-15 2021-02-13
2020 FB31 75.8 76.8 –0.19 34.4 83.3 59.1 24.5 5.6 2020-03-24 2021-02-14
2018 AK39 75.3 75.4 –0.01 27.3 75.4 51.4 25.3 6.5 2018-01-18 2021-02-18
2021 LL37 73.9 74.2 –0.05 36.1 74.6 55.4 22.7 4.0 2021-06-02 2022-05-31
2010 GB174 73.6 70.7 +0.54 48.7 630.7 339.7 25.3 6.5 2010-04-12 2013-04-30
2015 VJ168 73.4 72.4 +0.19 37.6 81.5 59.5 24.8 5.8 2015-11-06 2018-10-03
2015 DU249 73.1 72.7 +0.06 34.7 73.7 54.2 23.9 5.2 2015-02-17 2018-07-23
2014 FJ72 72.6 70.1 +0.46 38.4 148.2 93.3 24.4 5.6 2014-03-24 2016-08-31
2016 TS97[k] 71.2 71.5 −0.04 36.2 71.7 54.0 24.9 6.1 2016-10-06 2018-04-02
2015 GN55 71.0 72.1 −0.19 32.5 78.4 55.5 24.6 5.8 2015-04-13 2018-09-02
2015 VL168 69.7 72.1 –0.44 37.7 136.0 86.8 24.7 6.1 2015-11-07 2018-10-03
2020 BA95 69.6 68.4 +0.20 35.9 76.5 56.2 24.3 5.8 2020-01-25 2021-12-17
2015 RZ277 69.3 67.5 +0.32 34.7 90.5 62.6 25.6 6.8 2015-09-08 2018-10-01
2021 DJ17 69.0 69.2 40.4 69.4 54.9 23.2 6.7 2021-02-17 2022-05-31
2012 FH84 68.8 68.4 +0.07 41.9 70.1 56.0 25.8 7.2 2012-03-25 2016-06-07
2019 AC77 68.7 69.9 –0.21 35.0 79.0 57.0 25.0 6.6 2019-01-11 2021-02-14
2015 GR50 68.6 68.2 +0.07 38.2 69.7 54.0 25.2 6.6 2015-04-13 2016-08-31
2013 FQ28 68.4 67.3 +0.19 45.6 80.0 62.7 24.5 6.0 2013-03-17 2016-06-07
2011 GM89 68.3 68.5 –0.24 36.5 68.8 52.7 25.7 7.1 2011-04-04 2016-08-31
2021 DQ15 68.3 71.4 27.8 130.9 79.3 24.7 6.3 2021-02-16 2021-12-17
2021 DG17 67.6 66.7 +0.15 47.5 75.8 61.7 23.2 5.0 2021-02-17 2022-05-31
2015 GP50 67.5 68.1 –0.10 40.4 70.0 55.2 25.0 6.5 2015-04-14 2016-06-07
2016 CD289 67.2 66.2 +0.18 37.5 74.0 55.8 25.7 7.3 2016-02-05 2018-03-13
2018 VJ137 67.2 69.7 –0.42 37.8 139.3 88.5 25.2 6.9 2018-01-15 2021-02-13
2020 KV11 67.1 64.1 +0.50 35.0 155.0 95.6 25.6 7.3 2020-05-29 2022-11-02
2014 UD228 66.7 65.7 +0.18 36.7 73.3 55.0 24.5 6.1 2014-10-22 2017-12-07
2016 GB277 66.2 68.3 –0.39 40.0 119.4 79.7 25.6 7.3 2016-04-10 2020-06-04
2016 GZ276 66.1 69.2 –0.56 38.6 253.6 146.1 25.3 7.0 2016-04-10 2020-06-03
2014 FL72 66.1 63.3 +0.47 38.0 167.1 102.5 25.1 6.8 2014-03-26 2016-08-31
2016 TQ120[k] 65.8 63.7 +0.37 42.3 114.3 78.3 25.0 6.7 2016-10-06 2020-06-04
2015 RQ281 65.7 62.7 +0.56 36.9 210.6 123.8 25.1 6.8 2015-09-05 2019-03-27
2020 BS60[k] 65.7 68.0 –0.42 31.0 104.1 67.6 24.6 6.5 2020-01-26 2021-02-23
2013 UJ15 65.4 64.8 +0.11 37.2 67.4 52.3 25.4 7.0 2013-10-28 2016-08-31
2019 EV5 65.3 63.5 +0.30 32.0 79.8 55.9 25.8 7.6 2020-03-05 2021-12-17
2014 FD70 65.2 63.8 +0.26 35.9 78.6 57.3 25.1 6.9 2014-03-25 2018-04-02
2018 AZ18 65.1 65.9 –0.15 39.1 70.5 54.8 26.0 7.7 2018-01-15 2019-03-27
2015 KV167 65.0 65.2 –0.03 38.0 65.3 51.6 25.6 7.2 2015-05-18 2018-03-13
2018 VO35 65.0 67.8 –0.51 35.2 152.2 93.7 24.9 6.8 2018-11-10 2019-02-10
2020 KX11[k] 65.0 65.0 –0.01 64.6 67.1 65.9 26.4 8.2 2020-05-29 2020-09-25
This table includes all observable objects currently located at least 65 AU from the Sun.[18]


Size, mass and density

Size estimates
Year Radius (diameter) Source
2005 1,199 (2,397) km[54] Hubble
2007 1,300 (2,600) km[55] Spitzer
2011 1,163 (2,326) km[8] Occultation

In 2005, the diameter of Eris was measured to be 2397±100 km, using images from the Hubble Space Telescope (HST).[54][56] The size of an object is determined from its absolute magnitude (H) and the albedo (the amount of light it reflects). At a distance of 97 AU, an object with a diameter of 3,000 km would have an angular size of 40 milliarcseconds,[15] which is directly measurable with the Hubble Space Telescope. Although resolving such small objects is at the very limit of its capabilities,[l] sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.[m]

EarthMoonCharonCharonNixNixKerberosKerberosStyxStyxHydraHydraPlutoPlutoDysnomiaDysnomiaErisErisNamakaNamakaHi'iakaHi'iakaHaumeaHaumeaMakemakeMakemakeMK2MK2XiangliuXiangliuGonggongGonggongWeywotWeywotQuaoarQuaoarSednaSednaVanthVanthOrcusOrcusActaeaActaeaSalaciaSalacia2002 MS42002 MS4File:10 Largest Trans-Neptunian objects (TNOS).png
Artistic comparison of Pluto, Eris, Makemake, Haumea, Gonggong (2007 OR10), Sedna, Quaoar, Orcus, 2002 MS4, and Salacia.

This makes Eris around the same size as Pluto, which is 2372±4 km across. It also indicates an albedo of 0.96, higher than that of any other large body in the Solar System except Enceladus.[8] 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.[58]

In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope gave an estimate of Eris's diameter of 2600+400
−200 km.[55] The Spitzer and Hubble estimates overlap in the range of 2,400–2,500 km, 4–8% larger than Pluto. Astronomers now suspect that Eris's spin axis is currently pointing toward the Sun, which would make the sunlit hemisphere warmer than average and skew any infrared measurements toward higher values.[9] So the outcome from the 2010 Chile occultation is actually more in line with the Hubble result from 2005.[9]

In November 2010, Eris was the subject of one of the most distant stellar occultations yet from Earth.[9] Preliminary data from this event cast doubt on previous size estimates.[9] The teams announced their final results from the occultation in October 2011, with an estimated diameter of 2326+6
−6 km.[8] The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia's period—15.774 days—[10][59] Eris is 27 percent more massive than Pluto. If the 2011 occultation results are used, then Eris has a density of 2.52±0.07 g/cm3,[d] substantially denser than Pluto, and thus must be composed largely of rocky materials.[8]

Models of internal heating via radioactive decay suggest that Eris could have an internal ocean of liquid water at the mantle–core boundary.[60]

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. Eris is now the tenth-largest object known to directly orbit the sun by volume, though not by mass.[61]

Surface and atmosphere

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 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.[62] No surface details can be resolved from Earth or its orbit with any instrument currently available.

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.[63]

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.[64]

Satellite

Main article: Dysnomia (moon)

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.[65] 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 TV by Lucy Lawless.[66]

Exploration

It was calculated that a flyby mission to Eris could take 24.66 years using a Jupiter gravity assist, based on launch dates of 3 April 2032 or 7 April 2044. Eris would be 92.03 or 90.19 AU from the Sun when the spacecraft arrives.[67]

See also

Template:Wikipedia books

Notes

  1. ^ Images were taken on October 21, 2003; the object was not detected and identified until 2005.
  2. ^ [3] Both are common; the former is the literary pronunciation of the name, but Brown and his students use the latter.[4]
  3. ^ a b Calculated from the mean radius
  4. ^ a b Calculated by dividing the listed mass by the listed volume
  5. ^ a b Calculated based on the known parameters
  6. ^ Calculated from the listed diameter (D) and albedo (p) using [14]
  7. ^ As of February 2016, Sedna is 85.7 astronomical units (1.282×1010 km; 7.97×109 mi) from the Sun,[19] whereas Eris is 96.3 astronomical units (1.441×1010 km; 8.95×109 mi) from the Sun.[13] Eris is still close to its 1977 aphelion (furthest distance from the Sun), whereas Sedna is nearing its 2076 perihelion (closest approach to the Sun).[20] Sedna will overtake Eris as the farthest presently known spherical minor planet in 2114.[20]
  8. ^ Brown is joking on 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.[30]
  9. ^ For an example of an amateur image of Eris, see Fred Bruenjes' Astronomy
  10. ^ AU/yr indicates whether the object is moving inwards or outwards in its orbit, and the rate at which it does so.
  11. ^ a b c d e f g Cite error: The named reference shortarc was invoked but never defined (see the help page).
  12. ^ The Resolution of the High Resolution Channel of the ACS is 40 marcsec (milliarcseconds) and the size of 1 pixel is ~25 marcsec i.e. ~1875 km at the distance of Eris.
  13. ^ The reference to 'direct' measurement by the HST should not mislead into thinking that this method is as 'direct' and model-independent as measuring say Neptune's size. Basically, the method consists in finding the statistically best fit to a smeared image of the size of less than 2 pixels by comparing it with smeared images of the background stars, using a given computer model of the optics (PSF). A non-technical description of the method is given on Brown's page, a detailed description of this approach and its limitations are discussed in a paper on Quaoar[57]

References

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  3. ^ "Define Eris". Dictionary.com. Random House. Retrieved October 4, 2012.
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  20. ^ a b JPL Horizons On-Line Ephemeris System (July 18, 2010). "Horizons Output for Sedna 2076/2114". Archived from the original on February 25, 2012. Retrieved July 18, 2010. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help) Horizons
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  24. ^ Brown, Mike (November 22, 2010). "How big is Pluto, anyway?". Mike Brown's Planets. Retrieved November 23, 2010. (Franck Marchis on 2010-11-08)
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  28. ^ "International Astronomical Association homepage". Archived from the original on September 30, 2007. Retrieved January 5, 2007.
  29. ^ a b Green, Daniel W. E. (September 13, 2006). "(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia)" (PDF). IAU Circular. 8747. Archived from the original on February 5, 2007. Retrieved January 12, 2012. {{cite journal}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  30. ^ "The girl who named a planet". BBC News. January 13, 2006. Retrieved June 21, 2007.
  31. ^ "Xena and Gabrielle" (PDF). Status. January 2006. Retrieved May 3, 2007.
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  47. ^ Peat, Chris. "Spacecraft escaping the Solar System". Heavens-Above. Retrieved January 25, 2008.
  48. ^ Simulation of Eris (2003 UB313)'s orbit predicting a 17:5 resonance
  49. ^ "MPEC 2021-C187 : 2018 AG37". Minor Planet Electronic Circular. Minor Planet Center. February 10, 2021. Retrieved February 10, 2021.
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    Ephemeris Type: Vector; Observer Location: @sun; Time Span: Start=2015-12-01, Stop=2021-06-01, Intervals=1; Table Settings: quantities code=6
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  65. ^ Brown, M. E.; Van Dam, M. A.; Bouchez, A. H.; Le Mignant, D.; Campbell, R. D.; Chin, J. C. Y.; Conrad, A.; Hartman, S. K.; Johansson, E. M.; Lafon, R. E.; Rabinowitz, D. L. Rabinowitz; Stomski, P. J., Jr.; Summers, D. M.; Trujillo, C. A.; Wizinowich, P. L. (2006). "Satellites of the Largest Kuiper Belt Objects" (PDF). The Astrophysical Journal. 639 (1): L43–L46. arXiv:astro-ph/0510029. Bibcode:2006ApJ...639L..43B. doi:10.1086/501524. Retrieved October 19, 2011.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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External links

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