Eris (dwarf planet)

Eris (dwarf planet)

Overview
Eris, formal designation
Minor planet names
Formal minor planet designations are number–name combinations overseen by the Minor Planet Center, a branch of the IAU. They are used for dwarf planets and small Solar System bodies such as asteroids, but not comets...

 136199 Eris, is the most massive known dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 and the ninth most massive body known to orbit the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 directly. It is estimated to be approximately 2300–2400 km in diameter, and 27% more mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

ive than Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 or about 0.27% of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's mass.

Eris was discovered in January 2005 by a Palomar Observatory
Palomar Observatory
Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...

-based team led by Mike Brown
Michael E. Brown
Michael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003....

, and its identity was verified later that year. It is a trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (TNO) and a member of a high-eccentricity population known as the scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

.
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Encyclopedia
Eris, formal designation
Minor planet names
Formal minor planet designations are number–name combinations overseen by the Minor Planet Center, a branch of the IAU. They are used for dwarf planets and small Solar System bodies such as asteroids, but not comets...

 136199 Eris, is the most massive known dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 and the ninth most massive body known to orbit the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 directly. It is estimated to be approximately 2300–2400 km in diameter, and 27% more mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

ive than Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

 or about 0.27% of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's mass.

Eris was discovered in January 2005 by a Palomar Observatory
Palomar Observatory
Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...

-based team led by Mike Brown
Michael E. Brown
Michael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003....

, and its identity was verified later that year. It is a trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (TNO) and a member of a high-eccentricity population known as the scattered disc
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

. It has one known moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

, Dysnomia
Dysnomia (moon)
- References :...

. As of 2011, its distance from the Sun is 96.6 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

, roughly three times that of Pluto. With the exception of some comets, Eris and Dysnomia are currently the most distant known natural objects in the Solar System.

Because Eris appeared to be larger than Pluto, its discoverers and NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 initially described it as the Solar System’s tenth planet
Planets beyond Neptune
Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X...

. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 (IAU) to define the term planet
Definition of 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 objects which apparently move over the sky...

 for the first time. Under the IAU definition approved on August 24, 2006, Eris is a "dwarf planet" along with Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, Ceres, Haumea and Makemake.

In 2010, preliminary results from observations of a stellar occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

 by Eris on November 6 suggested that its diameter may be only 2,326 km, which would make it essentially the same size as Pluto. Given the error bar
Error bar
Error bars are a graphical representation of the variability of data and are used on graphs to indicate the error, or uncertainty in a reported measurement. They give a general idea of how accurate a measurement is, or conversely, how far from the reported value the true value might be...

s in the different size estimates, it is currently uncertain whether Eris or Pluto has the larger diameter. Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km.

Discovery


Eris was discovered by the team of Mike Brown
Michael E. Brown
Michael E. Brown has been a professor of planetary astronomy at the California Institute of Technology since 2003....

, Chad Trujillo
Chad Trujillo
Chadwick A. "Chad" Trujillo is an astronomer and the co-discoverer of the dwarf planet Eris.Trujillo works with computer software and has examined the orbits of the numerous trans-Neptunian objects , which is the outer area of the solar system that he specialized in. In late August 2005, it was...

, and David Rabinowitz
David L. Rabinowitz
David Lincoln Rabinowitz is a researcher at Yale University. He has built CCD cameras and software for the detection of near-Earth asteroids and Kuiper Belt Objects, and his research has helped reduce the assumed number of near-Earth asteroids by half, from 1,000-2,000 to 500-1,000 He has also...

 on January 5, 2005, from images taken on October 21, 2003. The discovery was announced on July 29, 2005, the same day as and two days after . The search team had been systematically scanning for large outer solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 bodies for several years, and had been involved in the discovery of several other large TNOs, including 50000 Quaoar
50000 Quaoar
50000 Quaoar is a rocky trans-Neptunian object in the Kuiper belt with one known moon. Discovered on June 4, 2002 by astronomers Chad Trujillo and Michael Brown at the California Institute of Technology from images acquired at the Samuel Oschin Telescope at Palomar Observatory, it is thought by...

, 90482 Orcus
90482 Orcus
90482 Orcus is a trans-Neptunian object in the Kuiper belt with a large moon. It was discovered on February 17, 2004 by Michael Brown of Caltech, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University. Precovery images as early as November 8, 1951 were later identified...

, and 90377 Sedna
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

.

Routine observations were taken by the team on October 21, 2003, using the 1200 mm Samuel Oschin
Samuel Oschin telescope
The Samuel Oschin telescope is a 48-inch aperture Schmidt camera at the Palomar Observatory in northern San Diego County, California. It consists of a 49.75-inch Schmidt corrector plate and a 72-inch mirror. The instrument is strictly a camera; there is no provision for an eyepiece to look...

 Schmidt telescope at Mount Palomar Observatory, California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

, 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 star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s.
Follow-up observations were then carried out to make a preliminary determination of Eris' orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

, which allowed the object's 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, , was announced by a different team in Spain.

More observations released in October 2005 revealed that Eris had a moon, later named Dysnomia
Dysnomia (moon)
- References :...

. Observations of Dysnomia's orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be , 27% greater than Pluto's.

Classification


Eris is classified as a plutoid, that is, a trans-Neptunian object
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 that is also a dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

. Its orbital characteristics more specifically categorize it a scattered disk object
Scattered disc
The scattered disc is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater...

 (SDO), or a TNO that is believed to have been "scattered" from the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 into more distant and unusual orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

s following gravitational interactions with Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

 as the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 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.

Because Eris may be larger than Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, it was initially described as the "tenth planet" by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 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
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

, the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 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 planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

s,
a category distinct from the new definition of planet.
Brown has since stated his approval of Pluto losing its status as a planet. The IAU subsequently added Eris to its Minor Planet Catalogue, designating it (136199) Eris.

Name



Eris is named after the Greek
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

 goddess Eris
Eris (mythology)
Eris is the Greek goddess of strife and discord, her name being translated into Latin as Discordia. Her Greek opposite is Harmonia, whose Latin counterpart is Concordia. Homer equated her with the war-goddess Enyo, whose Roman counterpart is Bellona...

 (Greek ), a personification of strife and discord.
The name was assigned on September 13, 2006, following an unusually long period in which the object was known by the provisional designation
Provisional designation in astronomy
Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calculated...

 , which was granted automatically by the IAU
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

 under their naming protocols for minor planet
Minor planet
An asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...

s. The regular adjectival form of Eris is Eridian.

Xena


Due to uncertainty over whether the object would be classified as a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 or a minor planet
Minor planet
An asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...

, as 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 eponymous heroine of the television series Xena: Warrior Princess
Xena: Warrior Princess
Xena: Warrior Princess is an American–New Zealand supernatural fantasy adventure series that aired in syndication from September 4, 1995 until June 18, 2001....

. The discovery team had reportedly saved the nickname "Xena" for the first body they discovered that was larger than Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

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

Choosing an official name


According to science writer Govert Schilling, Brown initially wanted to call the object "Lila", after a concept in Hindu mythology
Hindu mythology
Hindu religious literature is the large body of traditional narratives related to Hinduism, notably as contained in Sanskrit literature, such as the Sanskrit epics and the Puranas. As such, it is a subset of Nepali and Indian culture...

 that described the cosmos as the outcome of a game played by Brahma
Brahma
Brahma is the Hindu god of creation and one of the Trimurti, the others being Vishnu and Shiva. According to the Brahma Purana, he is the father of Mānu, and from Mānu all human beings are descended. In the Ramayana and the...

. 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
90377 Sedna
90377 Sedna is a trans-Neptunian object discovered in 2003, which was about three times as far from the Sun as Neptune. For most of its orbit it is even further from the Sun, with its aphelion estimated at 960 astronomical units , making it one of the most distant known objects in the Solar System...

 a year previously, and had been heavily criticised. However, 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
Controversy over the discovery of Haumea
was the first of all the dwarf planets to be discovered since Pluto in 1930. However, its naming and formal acceptance as a dwarf planet were delayed by several years due to controversy over who should receive credit for discovering it. A California Institute of Technology team headed by...

, 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
Persephone
In Greek mythology, Persephone , also called Kore , is the daughter of Zeus and the harvest-goddess Demeter, and queen of the underworld; she was abducted by Hades, the god-king of the underworld....

, the wife of the god Pluto
Pluto (mythology)
In ancient Greek religion and myth, Pluto was a name for the ruler of the underworld; the god was also known as Hades, a name for the underworld itself...

, would be a good name for the object. The name had been used several times in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

, and was popular with the public, having handily won a poll conducted by New Scientist
New Scientist
New Scientist is a weekly non-peer-reviewed English-language international science magazine, which since 1996 has also run a website, covering recent developments in science and technology for a general audience. Founded in 1956, it is published by Reed Business Information Ltd, a subsidiary of...

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
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

 with that name, 399 Persephone
399 Persephone
399 Persephone is a typical Main belt asteroid.It was discovered by Max Wolf on February 23, 1895 in Heidelberg.-External links:...

. Because IAU regulations
Astronomical naming conventions
In ancient times, only the Sun and Moon, a few hundred stars and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year...

 require a name from creation mythology for objects with orbital stability beyond Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

’s orbit, the team had also been considering such possibilities.

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, as the object had been considered a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 for so long, it deserved a name from Greek
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

 and Roman mythology
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...

, 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.

Orbit


Eris has an orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...

 of 557 years, and as of 2011 lies at 96.6 astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

s from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, almost its maximum possible distance (its aphelion
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

 is 97.5 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 probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

s. However, approximately forty known TNO
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

s, most notably , and , 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
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

, 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 plutino
Plutino
In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...

s, follows a less inclined and less eccentric orbit and, protected by orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

, 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. Unlike the eight planets, whose orbits all lie roughly in the same plane as the Earth's, Eris' orbit is highly inclined
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

: It is tilted at an angle of about 44 degrees
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

 to the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

. In about 800 years, Eris will be closer to the Sun than Pluto for some time (see the graph at the right).

Eris currently has an apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of 18.7, making it bright enough to be detectable to some amateur telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

s. A 200 mm telescope with a CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 can detect Eris under favourable conditions. The reason it had not been noticed until now is 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
Sculptor (constellation)
Sculptor is a small and faint constellation in the southern sky. It represents a sculptor. It was introduced by Nicolas Louis de Lacaille in the 18th century. He originally named it Apparatus Sculptoris , but the name was later shortened.-Notable features:No stars brighter than 3rd magnitude are...

 from 1876 until 1929 and Phoenix
Phoenix (constellation)
Phoenix is a minor constellation in the southern sky. It is named after the Phoenix, a mythical bird. It is faint: there are only two stars in the whole constellation which are brighter than magnitude 5.0...

 from roughly 1840 until 1875. In 2036 it will enter Pisces
Pisces (constellation)
Pisces is a constellation of the zodiac. Its name is the Latin plural for fish, and its symbol is . It lies between Aquarius to the west and Aries to the east...

 and stay there until 2065, when it will enter Aries
Aries (constellation)
Aries is one of the constellations of the zodiac, located between Pisces to the west and Taurus to the east. Its name is Latin for ram, and its symbol is , representing a ram's horns...

. It will then move into the northern sky
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

, entering Perseus
Perseus (constellation)
Perseus is a constellation in the northern sky, named after the Greek hero Perseus. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations defined by the International Astronomical Union...

 in 2128 and Camelopardalis
Camelopardalis
Camelopardalis is a large but faint constellation in the northern sky. The constellation was introduced in 1612 by Petrus Plancius. Some older astronomy books give an alternative spelling of the name, Camelopardus.-Etymology:...

 (where it will reach its northernmost declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

) in 2173. Because of the high inclination of its orbit, Eris only passes through a few constellations of the traditional Zodiac
Zodiac
In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...

.

Size, mass and density



Size estimates:
Year Radius (Diameter) Source
2005 Hubble
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

2007
Spitzer
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

2011 Occultation


In 2005, the diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 of Eris was measured to be 2,397 km, give or take 100 km, using images from the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 (HST). The size of an object is determined from its absolute magnitude (H) and the albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 (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 milliarcsecond
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

s, which is directly measurable with the Hubble Space Telescope. Although resolving such small objects is at the very limit of the telescope's capabilities, sophisticated image processing techniques such as deconvolution
Deconvolution
In mathematics, deconvolution is an algorithm-based process used to reverse the effects of convolution on recorded data. The concept of deconvolution is widely used in the techniques of signal processing and image processing...

 can be used to measure such angular sizes fairly accurately.

This makes Eris around the same size as Pluto, which is about 2,330 km across. It also indicates an albedo of 0.96, higher than that of any other large body in the Solar System except Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

. 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.

In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...

 gave an estimate of Eris's diameter of . The Spitzer and Hubble estimates overlap in the range of 2,400–2,500 km, 4–8% larger than Pluto. However, astronomers now suspect that Eris's spin axis is pointing toward the sun, at the moment—a possibility that would keep the sunlit hemisphere warmer than average and skew any infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 measurements toward higher values. So the outcome from the 2010 Chile occultation is actually more in line with the Hubble result from 2005.

In November 2010, Eris was the subject of one of the most distant stellar occultations yet achieved 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 . However, when using data from this event for comparison to Pluto, there is a range of figures available for Pluto's radius/diameter that can be selected. This is due in part to Pluto's atmosphere which interferes with making measurements of its solid surface (as opposed to gaseous haze).

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. If the 2011 occultation results are used, then Eris has a density of 2.52±0.05 g cm−3; substantially denser than Pluto, and thus must be composed largely of rocky materials.

Surface and atmosphere



The discovery team followed up their initial identification of Eris with spectroscopic
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...

 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
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 ice, indicating that the surface may be similar to that of Pluto, which at the time was the only TNO
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 known to have surface methane, and of Neptune's moon Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

, which also has methane on its surface. Note that no surface details can be resolved from Earth or its orbit with any instrument currently available.

Due to Eris's distant eccentric orbit, Eridian surface temperature is estimated to vary between about 30 and 56 kelvin (−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 tholin
Tholin
Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...

s 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
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

 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 sublime
Sublimation (physics)
Sublimation is the process of transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase...

. As methane is highly volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...

, 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
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

. This contrasts with observations of another recently discovered TNO, , which reveal the presence of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 ice but not methane.

Moon


In 2005, the adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

 team at the Keck telescopes
Keck telescopes
The W. M. Keck Observatory is a two-telescope astronomical observatory at an elevation of near the summit of Mauna Kea in Hawai'i. The primary mirrors of each of the two telescopes are in diameter, making them the second largest optical telescopes in the world, slightly behind the Gran Telescopio...

 in Hawaii carried out observations of the four brightest TNOs
Trans-Neptunian object
A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater distance on average than Neptune.The first trans-Neptunian object to be discovered was Pluto in 1930...

 (Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, , , and Eris), using the newly commissioned laser guide star
Laser guide star
Laser guide stars are a form of artificial star created for use in astronomical adaptive optics imaging.Adaptive optics systems require a wavefront reference source in order to correct atmospheric distortion of light...

 adaptive optics system. Images taken on September 10 revealed a moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 in orbit around Eris. In keeping with the "Xena" nickname already in use for Eris, Brown's team nicknamed the moon "Gabrielle
Gabrielle (Xena)
Gabrielle is a fictional character played by Renée O'Connor in Xena: Warrior Princess. She is referred to by fans as the Battling Bard of Potidaea. Her trademark weapons are the Amazon fighting staff and later, the sais...

", after the television warrior princess's sidekick. When Eris received its official name from the IAU, the moon received the name Dysnomia
Dysnomia (moon)
- References :...

,
after the Greek goddess of lawlessness
Dysnomia (mythology)
Dysnomia , imagined by Hesiod among the daughters of "abhorred Eris" , is the daemon of "lawlessness", who shares her nature with Atë ; she makes rare appearances among other personifications in poetical contexts that are marginal to Greek mythology but become central to Greek philosophy: see...

 who was Eris's daughter. The name also retains an oblique reference to Eris's old informal name Xena, portrayed on TV by Lucy Lawless
Lucy Lawless
Lucy Lawless, MNZM is a New Zealander actress and singer best known for playing the title character of the internationally successful television series Xena: Warrior Princess....

.

See also


  • Astronomical naming conventions
    Astronomical naming conventions
    In ancient times, only the Sun and Moon, a few hundred stars and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year...

  • Clearing the neighbourhood
    Clearing the neighbourhood
    "Clearing the neighbourhood of its orbit" is a criterion for a celestial body to be considered a planet in the Solar System. This was one of the three criteria adopted by the International Astronomical Union in its 2006 definition of planet....

  • Planets beyond Neptune
    Planets beyond Neptune
    Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X...

  • International Astronomical Union
    International Astronomical Union
    The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...


External links