Eris (center) and Dysnomia (left of center), taken by the Hubble Space Telescope
| |||||||||
| Discovery | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Discovered by | |||||||||
| Discovery date | October 21, 2003 | ||||||||
| Designations | |||||||||
| MPC designation | (136199) Eris | ||||||||
| Pronunciation | /ˈɪərɪs/ or /ˈɛrɪs/ | ||||||||
Named after
| Eris | ||||||||
| 2003 UB313 | |||||||||
| Adjectives | Eridian | ||||||||
| Orbital characteristics | |||||||||
| Epoch December 9, 2014 (JD 2457000.5) | |||||||||
| Earliest precovery date | September 3, 1954 | ||||||||
| Aphelion |
| ||||||||
| Perihelion |
| ||||||||
| |||||||||
| Eccentricity | 0.44068 | ||||||||
| |||||||||
Average orbital speed
| 3.4338 km/s | ||||||||
| 204.16° | |||||||||
| Inclination | 44.0445° | ||||||||
| 35.9531° | |||||||||
| 150.977° | |||||||||
| Known satellites | Dysnomia | ||||||||
| Physical characteristics | |||||||||
| Dimensions | 2326±12 km | ||||||||
Mean radius
| 1163±6 km | ||||||||
| (1.70±0.02)×107 km2 | |||||||||
| Volume | (6.59±0.10)×109 km3 | ||||||||
| Mass | |||||||||
Mean density
| 2.52±0.07 g/cm3 | ||||||||
Equatorial surface gravity
| 0.82±0.02 m/s2 0.083±0.002 g | ||||||||
Equatorial escape velocity
| 1.38±0.01 km/s | ||||||||
Sidereal rotation period
| 25.9±0.5 hr | ||||||||
| 0.96+0.09 −0.04 | |||||||||
| |||||||||
| B−V=0.78, V−R=0.45 | |||||||||
| 18.7 | |||||||||
| −1.17+0.06 −0.11 | |||||||||
| 40 milli-arcsec | |||||||||
Eris (minor-planet designation 136199 Eris) is the most massive and second-largest (by volume) dwarf planet in the known 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 Eris, the Greek goddess of strife and discord. Eris is the ninth most massive object 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. Eris's mass is about 0.27% of the Earth mass, about 27% more than dwarf planet Pluto, although Pluto is slightly larger by volume.
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), roughly three times that of Pluto. With the exception of some long-period comets, Eris and Dysnomia are currently the most distant known natural objects in the Solar System.
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, 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, which was measured by New Horizons as 2,372 ± 4 kilometers (1,473.9 ± 2.5 mi) in July 2015.
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 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, 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 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.
Follow-up observations were then carried out to make a preliminary determination of Eris's orbit,
which allowed the object's distance to be estimated. The team had
planned to delay announcing their discoveries of the bright objects Eris
and Makemake
until further observations and calculations were complete, but
announced them both on July 29 when the discovery of another large TNO
they had been tracking, Haumea, was controversially announced on July 27 by a different team in Spain.
Precovery images of Eris have been identified back to September 3, 1954.
More observations released in October 2005 revealed that Eris has a moon, later named Dysnomia. Observations of Dysnomia's orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be (1.66±0.02)×1022 kg, 27%±2% greater than Pluto's.
Name
Initially,
Eris was given a provisional designation, the so called "license plate"
name and then eventually a name based on Earth mythology.
Eris is named after the Greek goddess Eris (Greek Ἔρις), a personification of strife and discord. The name was proposed by the Caltech Team on September 6, 2006, and it 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
Artistic vision of Eris and its moon
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, the decision on what to name the object had to wait until after the August 24, 2006, IAU ruling. 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?), 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! | ” |
"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] 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. | ” |
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.
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.
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.
Brown had also 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 New Scientist magazine ("Xena", despite only being a nickname, came fourth). This was not possible once the object was classified as a dwarf planet, because there is already an asteroid with that name, 399 Persephone.
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. 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.
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). 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.
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 planetary searches
have traditionally 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. 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 this classification. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.
Orbit
Eris has an orbital period of 558 years. Its maximum possible distance from the Sun (aphelion) is 97.65 AU, and its closest (perihelion) is 37.91 AU. It came to perihelion between 1698 and 1699, to 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.
As of 2008 there were approximately forty known TNOs, most notably 2006 SQ372, 2000 OO67 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).
The distances of Eris and Pluto from the Sun in the next 1,000 years
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. 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 left).
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.
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.
Seen from earth, Eris makes small loops in the sky through the constellation of Cetus
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. 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
| Year | Radius (diameter) | Source |
|---|---|---|
| 2005 | 1,199 (2,397) km | Hubble |
| 2007 | 1,300 (2,600) km | Spitzer |
| 2011 | 1,163 (2,326) km | Occultation |
In November 2010, Eris was the subject of one of the most distant stellar occultations yet from Earth. Preliminary data from this event cast doubt on previous size estimates. The teams announced their final results from the occultation in October 2011, with an estimated diameter of 2326+6
−6 km.
−6 km.
This makes Eris a little smaller than 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.
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.
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. Using the 2011 occultation results, Eris has a density of 2.52±0.07 g/cm3, substantially denser than Pluto, and thus must be composed largely of rocky materials.
Models of internal heating via radioactive decay suggest that Eris could have an internal ocean of liquid water at the mantle–core boundary.
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, but remains the ninth-largest by mass.
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. 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).
Unlike the somewhat reddish Pluto and Triton, Eris appears almost white. 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.
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.
Satellite Dysnomia
In 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris), using the newly commissioned laser guide star adaptive optics system. Images taken on September 10 revealed a moon in orbit around Eris. In keeping with the "Xena" nickname already in use for Eris, Brown's team nicknamed the moon "Gabrielle",
after the television warrior princess' 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.
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.
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.
