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Eris (dwarf planet)

Related subjects Space (Astronomy)

Eris (centre) and Dysnomia (left of centre), taken by the Hubble Space Telescope.
Eris (centre) and Dysnomia (left of centre).
Hubble Space Telescope.
Discovered by M. E. Brown,
C. A. Trujillo,
D. L. Rabinowitz
Discovery date October 21, 2003
MPC designation 136199 Eris
Alternate name 2003 UB313
Minor planet
dwarf planet,
and SDO
Orbital characteristics
Epoch March 6, 2006
( JD 2453800.5)
Aphelion 97.56  AU
Perihelion 37.77 AU
5.65×109 km
Semi-major axis 67.6681 AU
10.12×109 km
Eccentricity 0.44177
Orbital period 203,600 days
557  years
Average orbital speed 3.436 km/s
Mean anomaly 197.63427°
Inclination 44.187°
Longitude of ascending node 35.8696°
Argument of perihelion 151.4305°
Satellites 1
Physical characteristics
Mean radius 1,300 +200 -100 km
Mass (1.67 ± 0.02)×1022 kg
Equatorial surface gravity ~0.8 m/s²
Sidereal rotation
> 8 h?
Albedo 0.86 ± 0.07
Surface temp.
min mean max
30  K 42.5 K 55 K
Apparent magnitude 18.7
Absolute magnitude −1.12 ± 0.01
Angular diameter 40 milli-arcsec

Eris (pronounced /ˈɪərɪs/, also /ˈɛrɪs/ as in Greek Έρις), formally designated 136199 Eris and prior to that 2003 UB313 (see minor planet names), is the largest known dwarf planet in the Solar System and the ninth largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto.

Eris was first spotted in 2003 by a Palomar Observatory-based team led by Mike Brown but not identified until 2005. It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc. Eris has one moon, Dysnomia; recent observations have found no evidence of further satellites. The current distance from the Sun is 96.7 AU, roughly three times that of Pluto. With the exception of some comets the pair are the most distant known natural objects in the Solar System.

Because Eris is larger than Pluto, its discoverers and NASA called it the Solar system’s tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term "planet" for the first time. Under a new definition approved on August 24, 2006, Eris was designated a "dwarf planet" along with Pluto and Ceres.


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 Makemake and two days after (136108) 2003 EL61. 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 1200 mm Samuel Oschin reflecting telescope at Mount 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' slow motion against the background stars.

Animation showing the movement of Eris on the images used to discover it. Eris is located on the left side, slightly above the middle of the image. The three frames were taken over a period of three hours.
Animation showing the movement of Eris on the images used to discover it. Eris is located on the left side, slightly above the middle of the image. The three frames were taken over a period of three hours.

Follow-up observations were then carried out to make a preliminary determination of its orbit, which allowed its distance to be estimated. The team had planned to delay announcing their discovery until further observations allowed more accurate calculations of Eris' orbit, but brought their announcement forward when the discovery of another large TNO they had been tracking, 2003 EL61, was announced by a different team in Spain.

More observations released in October 2005 revealed that Eris had 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, 27% greater than Pluto.


Eris is classified as a dwarf planet and trans-Neptunian object (TNO); the intersection of these categorizations makes it a " plutoid". Eris' orbital characteristics can be used to more specifically categorize it a scattered disk object (SDO), or a TNO that is believed to have 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. 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.

Distribution of trans-Neptunian Objects.
Distribution of trans-Neptunian Objects.

As Eris is larger than Pluto, it was initially 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 24 August 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 the "dwarf planet" label. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.


Athenian painting of Eris, circa 550 BC
Athenian painting of Eris, circa 550 BC

Eris is named after the goddess Eris ( Greek Έρις), a personification of strife and discord. The name was assigned on September 13, 2006 following an unusually long period in which it was known by the provisional designation 2003 UB313, which was granted automatically by the IAU under their naming protocols for minor planets. The adjectival form of Eris is Eridian.


Due to uncertainty over whether the object would be classified as a planet or a minor planet, as different nomenclature procedures apply to these different classes of object, the decision on what to name the object had to wait until after the August 24, 2006 IAU ruling. As a result, a number of interim names were used in the media between 2005 and 2006, in particular Xena and Lila.

“Xena” was an informal name used internally by the discovery team. It was inspired by the eponymous heroine 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?), and (this part is actually true) we've been working to get more female deities out there (i.e. Sedna). Also at the time the TV show was still on TV, which shows you how long we've been searching!

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

"One reporter 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."

The nickname "Lila" has also been used by the media. However, this was a misunderstanding of planetlila, the URL path of the discovery web page. The URL path was named after Mike Brown's then-newborn daughter, Lilah.

Choosing an official name

Brown had previously speculated that Persephone, the wife of the god Pluto, would be a good name for the object. The name had been used several times in science fiction, and was popular with the public, having handily won a poll conducted by NewScientist magazine ("Xena", despite only being a nickname, came fourth). However, this was not possible once the object was classified as a dwarf planet, because there is already an asteroid with that name, 399 Persephone. Because IAU regulations require a name from creation mythology for objects with orbital stability beyond Neptune’s orbit, the team had also been considering such possibilities.

With the dispute resolved, the discovery team proposed Eris on 6 September 2006. On 13 September 2006 it was accepted as the official name by the IAU. Brown decided that, as the object had been considered a planet for so long, it deserved a name from Graeco-Roman mythology, like the other planets. However, the asteroids had taken the vast majority of Graeco-Roman names. Eris, whom Brown described as his favourite goddess, had fortunately escaped inclusion. The name in part reflects the discord in the astronomical community caused by the debate over the object’s (and Pluto’s) nature, while the name of its moon, Dysnomia (“lawlessness”), retains an oblique reference to the dwarf planet’s old informal name Xena, portrayed on TV by Lucy Lawless.


The orbit of Eris (blue) compared to those of Saturn, Uranus, Neptune, and Pluto (white/grey). The arcs below the ecliptic are plotted in darker colours, and the red dot is the Sun. The diagram on the left is a polar view while the diagrams on the right are different views from the ecliptic.
The orbit of Eris (blue) compared to those of Saturn, Uranus, Neptune, and Pluto (white/grey). The arcs below the ecliptic are plotted in darker colours, and the red dot is the Sun. The diagram on the left is a polar view while the diagrams on the right are different views from the ecliptic.

Eris has an orbital period of 557 years, and as of 2008 lies at 96.7  astronomical units from the Sun, almost its maximum possible distance. (Its aphelion is 97.5 AU.) Eris came to perihelion between 1698-1699, aphelion around 1977, and will return to perihelion around 2256 to 2258. Eris and its moon are currently the most distant known objects in the Solar system apart from long-period comets and space probes. However, approximately forty known TNOs, most notably 2000 OO67 and Sedna, while currently closer to the Sun than Eris, have greater average orbital distances than Eris' semimajor axis of 67.7 AU.

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 (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. Unlike the terrestrial planets and gas giants, whose orbits all lie roughly in the same plane as the Earth's, Eris' orbit is highly inclined: It is tilted at an angle of about 44 degrees to the ecliptic.

Eris currently has an apparent magnitude of 18.7, making it bright enough to be detectable to some amateur telescopes. A 200 mm telescope with a CCD can detect Eris under favorable conditions. The reason it had not been noticed until now is because of its steep orbital inclination; most searches for large outer Solar system objects concentrate on the ecliptic plane, where most bodies are found.

Eris 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. It will then move into the northern sky, entering Perseus in 2128 and Camelopardalis (where it will reach its northernmost declination) in 2173. Because the orbit of Eris is highly inclined, it only passes through a few constellations of the traditional Zodiac.

Size, mass, and density

The diameter of Eris has been measured to be 2397 km, give or take 100 km, using images from the Hubble Space Telescope. (The brightness of an object depends both on its size and its albedo, the amount of light it reflects). At a distance of 97 AU, an object with a radius of 3000 km would have an angular size of 40  milliarcseconds, which is directly measurable with the HST; although resolving such small objects is at the very limit of Hubble's capabilities, sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.)

The Earth (136199) Eris (134340) Pluto (90377) Sedna

Eris compared to Pluto, Makemake, (136108) 2003 EL61, Sedna, Orcus, Quaoar, Varuna, and Earth.
Eris compared to Pluto, Makemake, (136108) 2003 EL61, Sedna, Orcus, Quaoar, Varuna, and Earth.

This makes Eris only 0-8% larger than Pluto, which is about 2306 km across. It also indicates an albedo of 0.86, higher than any other large body in the Solar system other than Enceladus. It is speculated that the high albedo is due to the surface ices being replenished due to temperature fluctuations as Eris' eccentric orbit takes it closer and farther from the Sun.

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. The Spitzer and Hubble estimates overlap in the range of 2400-2500 km, 4-8% larger than Pluto.

The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia's period, 15.774 days, Eris is 27 percent more massive than Pluto.

Thermal measurement

Previous observations of the thermal emission of Eris at a wavelength of 1.2 mm, where the object's brightness depends only on temperature and surface area, indicated a diameter of 3000 ± 400 km, about a third larger than Pluto. If the object rotates quickly, resulting in a more even heat distribution and a temperature of 23 to 24  kelvins (-250 to -249 degrees Celsius), a likely diameter would be in the higher portion of the range (best fit 3090 km); if it rotates slowly, the visible surface would be warmer (about 27 K, or -246 degrees Celsius) and a likely diameter would be in the smaller end of the range (best fit 2860 km). The 2860 km figure implies a Pluto-like albedo of 60%, consistent with its Pluto-like spectral signature.

The apparent inconsistency of the HST PSF results (2400 ± 100 km) with the above IRAM results (3000 ± 370 km) will certainly be studied at more length. Brown explains it by a slightly lower absolute magnitude than the one assumed by Bertoldi (−1.12 ± 0.01 versus −1.16 ± 0.1, resulting by itself in almost 100 km difference in diameter). Assuming further the highest diameter (2500 km) and pole-on position of the object the difference between the results would appear consistent with 1.1-σ error margin.

Another possible explanation for the IRAM results is offered by the Max-Planck-Institut für Radioastronomie. The ratio between the bolometric albedo (representing the total reflected energy and used in the thermal method) and the geometric albedo (representing the reflection in some visual wavelength and used to calculate the diameter from HST pictures) is not known with high precision and depends on many factors. By itself, this uncertainty could bridge the gap between the two measures.

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 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.

Due to its distant eccentric orbit, Eridian surface temperatures are estimated to vary between about 30 and 56 kelvins (−243 and −217 degrees Celsius).

Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey. Pluto's reddish colour is believed 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 away 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.

Even though Eris can be up to three times further from the Sun than Pluto, it approaches close enough that some of the ices on the surface might warm enough to sublimate. Methane is highly volatile and 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 it has an internal source of methane to replenish gas that escapes from its atmosphere. This contrasts with observations of another recently-discovered TNO, 2003 EL61, which reveal the presence of water ice but not methane.


Artist impression of Eris and Dysnomia. Eris is the main object, Dysnomia the small grey disk just above it. The flaring object top-left is the Sun.
Artist impression of Eris and Dysnomia. Eris is the main object, Dysnomia the small grey disk just above it. The flaring object top-left is the Sun.

In 2005 the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, Makemake, 2003 EL61, 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 demon of lawlessness who was Eris' daughter.

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