Dwarf planet

From Wikipedia, the free encyclopedia

Five recognized dwarf planets

Ceres (1801)			Pluto (1930)
Eris (2005)		Makemake (2005)		Haumea (2004)
The five dwarf planets: Ceres as seen from the Dawn spacecraft. It is the only dwarf planet in the asteroid belt. Pluto as viewed by New Horizons space probe on July 13, 2015. Eris and its moon Dysnomia viewed by the Hubble Space Telescope. Makemake and its moon S/2015 (136472) 1 viewed by the Hubble Space Telescope. NASA artist's impression of Haumea.

A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. That is, it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit.^[1]

The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,^[1] brought about by an increase in discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris.^[2] The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by astronomer Mike Brown,^[3][4][5] who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern,^[6][7] who had coined the term dwarf planet in April 1991.^[8]

As of July 2008 the International Astronomical Union (IAU) recognizes five dwarf planets: Ceres in the asteroid belt, and Pluto, Haumea, Makemake, and Eris in the outer solar system.^[9] Brown criticizes this official recognition: "A reasonable person might think that this means that there are five known objects in the solar system which fit the IAU definition of dwarf planet, but this reasonable person would be nowhere close to correct."^[10]

Another hundred or so known objects in the Solar System are suspected to be dwarf planets.^[11] Estimates are that up to 200 dwarf planets will be identified when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.^{[12][dead link]} Individual astronomers recognize several of these,^[11] and Brown maintains a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets.^[10] As of 5 Feb 2018, Brown's list includes 952 objects, identifying ten known trans-Neptunian objects—the four accepted by the IAU plus 2007 OR₁₀, Quaoar, Sedna, Orcus, (307261) 2002 MS4 and Salacia—as "near certain", with another 27 "highly likely."^[13] Stern states that there are more than a dozen known dwarf planets.^[12]

Only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto. They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1)^[14] are to be named under the assumption that they are dwarf planets.^[15]

The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.^[16]

History of the concept

Pluto and its moon Charon

4 Vesta, one of the largest asteroids

Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for decades considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid for the smaller bodies and then stopped naming or classifying them as planets.^[17]

With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies (asteroids and comets). For almost 50 years Pluto was thought to be larger than Mercury,^[18][19] but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates.^[20] It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had one-fifth the mass of Earth's Moon.^[21] Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets.^[22]

In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the Kuiper belt), and some even farther away.^[23] Many of these shared several of Pluto's key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects, plutinos. This led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the bodies now known as dwarf planets.^[24][25] By 2005, three trans-Neptunian objects comparable in size to Pluto (Quaoar, Sedna, and Eris) had been reported.^[26] It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified.^[27] Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain a planet.^[28]

Eris (then known as 2003 UB₃₁₃) was discovered in January 2005;^[29] it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet.^[30] As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006.^[31] The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by Uruguayan astronomer Julio Ángel Fernández: he proposed an intermediate category for objects large enough to be round but which had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits.^[32]

The IAU's final Resolution 5A preserved this three-category system for the celestial bodies orbiting the Sun. It reads:

The IAU ... resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
(1) A planet¹ is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,² (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects,³ except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies."

Footnotes:

¹ The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

² An IAU process will be established to assign borderline objects either dwarf planet or other status.

³ These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Although concerns were raised about the classification of planets orbiting other stars,^[16] the issue was not resolved; it was proposed instead to decide this only when such objects start to be observed.^[32]

Name

Euler diagram showing the types of bodies in the Solar System (except the Sun).

The term dwarf planet has itself been somewhat controversial, as it could imply that these bodies are planets, much as dwarf stars are stars. This is the conception of the Solar System that Stern promoted when he coined the phrase. The older word planetoid ("having the form of a planet") has no such connotation, and is also used by astronomers for bodies that fit the IAU definition.^[33] Brown states that planetoid is "a perfectly good word" that has been used for these bodies for years, and that the use of the term dwarf planet for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution.^[34] Indeed, the draft of Resolution 5A had called these median bodies planetoids,^[35][36] but the plenary session voted unanimously to change the name to dwarf planet.^[1] The second resolution, 5B, defined dwarf planets as a subtype of planet, as Stern had originally intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight classical planets and four dwarf planets. Resolution 5B was defeated in the same session that 5A was passed.^[34] Because of the semantic inconsistency of a dwarf planet not being a planet due to the failure of Resolution 5B, alternative terms such as nanoplanet and subplanet were discussed, but there was no consensus among the CSBN to change it.^[37]

In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French planète naine, Spanish planeta enano, German Zwergplanet, Russian karlikovaya planeta (карликовая планета), Arabic kaukab qazm (كوكب قزم), Chinese ǎixíngxīng (矮行星), Korean waesohangseong (왜소행성; 矮小行星), but in Japanese they are called junwakusei (準惑星) meaning "subplanets" or "almost-planets".

IAU Resolution 6a of 2006^[38] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects but neither name was carried forward, perhaps due to objections from geologists that this would create confusion with their pluton.^[1]

On June 11, 2008, the IAU Executive Committee announced a name, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids.^[15] The authority of that initial announcement has not been universally recognized:

"...in part because of an email miscommunication, the WG-PSN [Working Group for Planetary System Nomenclature] was not involved in choosing the word plutoid. ... In fact, a vote taken by the WG-PSN subsequent to the Executive Committee meeting has rejected the use of that specific term..."^[39]

Characteristics

Planetary discriminants^[40]

Body	M⊕ (1)	Λ (2)	µ (3)	Π (4)
Mercury	0.055	1.95 × 103	9.1 × 104	1.3 × 102
Venus	0.815	1.66 × 105	1.35 × 106	9.5 × 102
Earth	1	1.53 × 105	1.7 × 106	8.1 × 102
Mars	0.107	9.42 × 102	1.8 × 105	5.4 × 101
Ceres	0.00015	8.32 × 10−4	0.33	4.0 × 10−2
Jupiter	317.7	1.30 × 109	6.25 × 105	4.0 × 104
Saturn	95.2	4.68 × 107	1.9 × 105	6.1 × 103
Uranus	14.5	3.85 × 105	2.9 × 104	4.2 × 102
Neptune	17.1	2.73 × 105	2.4 × 104	3.0 × 102
Pluto	0.0022	2.95 × 10−3	0.077	2.8 × 10−2
Eris	0.0028	2.13 × 10−3	0.10	2.0 × 10−2
Sedna	0.00022	3.64 × 10−7	<0 .07="" class="reference" id="cite_ref-41" sup="">[41]

1.6 × 10⁻⁴
Showing the planets and the largest known sub-planetary objects (purple) covering the orbital zones containing likely dwarf planets. All known possible dwarf planets have smaller discriminants than those shown for that zone.

(1)	M⊕ Earth mass is the unit of mass equal to that of Earth (5.97 × 1024 kg).
(2)	Λ is the capacity to clear the neighbourhood (greater than 1 for planets) by Stern and Levison. Λ = k M2 a−3/2, where k = 0.0043 for units of Yg and AU, and a is the body's semi-major axis.[42]
(3)	µ is Soter's planetary discriminant (greater than 100 for planets). µ = M/m, where M is the mass of the body, and m is the aggregate mass of all the other bodies that share its orbital zone.
(4)	Π is the capacity to clear the neighbourhood (greater than 1 for planets) by Margot. Π = k M a−9/8, where k = 807 for units of Earth masses and AU.[43]

Orbital dominance

Alan Stern and Harold F. Levison introduced a parameter Λ (lambda), expressing the likelihood of an encounter resulting in a given deflection of orbit.^[42] The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. This value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit, where Λ > 1 will eventually clear it. A gap of five orders of magnitude in Λ was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects.^[40]
Using this parameter, Steven Soter and other astronomers argued for a distinction between planets and dwarf planets based on the inability of the latter to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that prevent collisions), whereas dwarf planets lack the mass to do so.^[42] Soter went on to propose a parameter he called the planetary discriminant, designated with the symbol µ (mu), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone), where µ > 100 is deemed to be cleared.^[40]

Jean-Luc Margot refined Stern and Levison's concept to produce a similar parameter Π (Pi).^[43] It is based on theory, avoiding the empirical data used by Λ. Π > 1 indicates a planet, and there is again a gap of several orders of magnitude between planets and dwarf planets.

There are several other schemes that try to differentiate between planets and dwarf planets,^[6] but the 2006 definition uses this concept.^[1]

Hydrostatic equilibrium

Sufficient internal pressure, caused by the body's gravitation, will turn a body plastic, and sufficient plasticity will allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than a few kilometers are dominated by non-gravitational forces and tend to have an irregular shape. Larger objects, where gravitation is significant but not dominant, are "potato" shaped; the more massive the body is, the higher its internal pressure and the more rounded its shape, until the pressure is sufficient to overcome its internal compressive strength and it achieves hydrostatic equilibrium. At this point a body is as round as it is possible to be, given its rotation and tidal effects, and is an ellipsoid in shape. This is the defining limit of a dwarf planet.^[44]

When an object is in hydrostatic equilibrium, a global layer of liquid covering its surface would form a liquid surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. If the body does not rotate, it will be a sphere, but the faster it rotates, the more oblate or even scalene it becomes. If such a rotating body were to be heated until it melted, its overall shape would not change when liquid. The extreme example of a non-spherical body in hydrostatic equilibrium is Haumea, which is twice as long along its major axis as it is at the poles. If the body has a massive nearby companion, then tidal forces come into effect as well, distorting it into a prolate spheroid. An example of this is Jupiter's moon Io, which is the most volcanically active body in the Solar System due to effects of tidal heating. Tidal forces also cause a body's rotation to gradually become tidally locked, such that it always presents the same face to its companion. An extreme example of this is the Pluto–Charon system, where both bodies are tidally locked to each other. Earth's Moon is also tidally locked, as are many satellites of the gas giants.

The masses of the IAU-recognized dwarf planets plus Charon relative to the Moon. The mass of Makemake is a rough estimate. (See plutoid for a graph of several additional likely dwarf planets without Ceres.)

The upper and lower size and mass limits of dwarf planets have not been specified by the IAU. There is no defined upper limit, and an object larger or more massive than Mercury that has not "cleared the neighbourhood around its orbit" would be classified as a dwarf planet.^[45] The lower limit is determined by the requirements of achieving a hydrostatic equilibrium shape, but the size or mass at which an object attains this shape depends on its composition and thermal history. The original draft of the 2006 IAU resolution redefined hydrostatic equilibrium shape as applying "to objects with mass above 5×10²⁰ kg and diameter greater than 800 km",^[16] but this was not retained in the final draft.^[1]

Empirical observations suggest that the lower limit will vary according to the composition and thermal history of the object. For a body made of rigid silicates, such as the stony asteroids, the transition to hydrostatic equilibrium should occur at a diameter of approximately 600 km and a mass of 3.4×10²⁰ kg. For a body made of less rigid water ice, the limit should be about 320 km and 10¹⁹ kg.^[46] In the asteroid belt, Ceres is the only body that clearly surpasses the silicaceous limit (though it is actually a rocky–icy body), and its shape is an equilibrium spheroid. 2 Pallas and 4 Vesta are rocky and are just below the limit. Pallas, at 525–560 km and 1.85–2.4×10²⁰ kg, is "nearly round" but still somewhat irregular. Vesta, at 530 km and 2.6×10²⁰ kg, deviates from an ellipsoid shape primarily due to a large impact basin at its pole.

Dwarf planets and possible dwarf planets

Illustration of the relative sizes, albedos, and colours of the largest trans-Neptunian objects

Eight dwarves or Candidates and moon's orbits, shown at angle

Many trans-Neptunian objects (TNOs) are thought to have icy cores and therefore would require a diameter of perhaps 400 km (250 mi)—only about 3% of that of Earth—to relax into gravitational equilibrium.^[47] As of January 2015, about 150 known TNOs are considered potential dwarf planets, although only rough estimates of the diameters of most of these objects are available.^[11] A team is investigating thirty of these, and think that the number will eventually prove to be around 200 in the Kuiper belt, with thousands more beyond.^[47]

Recognized

The IAU has recognized five bodies as dwarf planets since 2008: Ceres, Pluto, Eris, Haumea, and Makemake.^[48] Ceres and Pluto are known to be dwarf planets through direct observation.^[49] Eris is recognized as a dwarf planet because it is more massive than Pluto (measurements by New Horizons indicate that Pluto's diameter is larger than that of Eris), whereas Haumea and Makemake qualify based on their absolute magnitudes.^[9][38] In relative distance from the Sun, the five are:

1. Ceres – discovered on January 1, 1801, 45 years before Neptune. Considered a planet for half a century before reclassification as an asteroid. Accepted as a dwarf planet by the IAU on September 13, 2006.
2. Pluto ♇ – discovered on February 18, 1930. Classified as a planet for 76 years. Reclassified as a dwarf planet by the IAU on August 24, 2006.
3. Haumea – discovered on December 28, 2004. Accepted by the IAU as a dwarf planet on September 17, 2008.
4. Makemake – discovered on March 31, 2005. Accepted by the IAU as a dwarf planet on July 11, 2008.
5. Eris – discovered on January 5, 2005. Called the "tenth planet" in media reports. Accepted by the IAU as a dwarf planet on September 13, 2006.

Proposed

Mike Brown considers an additional six trans-Neptunian objects to be "nearly certainly"^[11] dwarf planets with diameters at or above 900 kilometers. These objects are:

1. Quaoar – discovered on June 5, 2002
2. 2002 MS₄ – discovered on June 18, 2002
3. Sedna – discovered on November 14, 2003
4. Orcus – discovered on February 17, 2004
5. Salacia – discovered on September 22, 2004
6. 2007 OR₁₀ – discovered on July 17, 2007

Tancredi et al. advised the IAU to officially accept Orcus, Sedna and Quaoar. In addition, Gonzalo Tancredi considers the five TNOs Varuna, Ixion, 2003 AZ₈₄, 2004 GV₉, and 2002 AW₁₉₇ to be dwarf planets as well.^[49]

 Vesta, the next-most-massive body in the asteroid belt after Ceres, is roughly spherical, deviating mainly because of massive impacts that formed Rheasilvia and Veneneia crater after it solidified.^[55] Furthermore, its triaxial dimensions are not consistent with hydrostatic equilibrium.^[56][57] Triton is thought to be a captured dwarf planet.^[58] Phoebe is a captured body that, like Vesta, is no longer in hydrostatic equilibrium, but is thought to have been so early in its history.^[59]

Exploration

On March 6, 2015, the Dawn spacecraft began to orbit Ceres, becoming the first spacecraft to orbit a dwarf planet.^[60] On July 14, 2015, the New Horizons space probe flew by Pluto and its five moons. Dawn has also explored the former dwarf planet Vesta. Phoebe has been explored by Cassini (most recently) and Voyager 2, which also explored Triton. These three are thought to be former dwarf planets and therefore their exploration helps in the study of the evolution of dwarf planets.

Contention

In the immediate aftermath of the IAU definition of dwarf planet, some scientists expressed their disagreement with the IAU resolution.^[6] Campaigns included car bumper stickers and T-shirts.^[61] Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.^[62]

NASA has announced that it will use the new guidelines established by the IAU.^[63] Alan Stern, the director of NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets.^[64] Thus, in 2011, he still referred to Pluto as a planet,^[65] and accepted other dwarf planets such as Ceres and Eris, as well as the larger moons, as additional planets.^[66] Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both types "planets".^[42]

Planetary-mass moons

Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity, and seven of them are more massive than either Eris or Pluto. They are not physically distinct from the dwarf planets, but are not dwarf planets because they do not directly orbit the Sun. The seven that are more massive than Eris are the Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), one moon of Saturn (Titan), and one moon of Neptune (Triton). The others are six moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), and one moon of Pluto (Charon). There are additional possibilities among TNOs, including Dysnomia orbiting Eris. Alan Stern calls these moons "satellite planets", one of three categories of planet together with dwarf planets and classical planets.^[66] The term planemo ("planetary-mass object") covers all three.^[67]

In a draft resolution for the IAU definition of planet, both Pluto and Charon would have been considered dwarf planets in a binary system, given that they both satisfied the mass and shape requirements for dwarf planets and revolved around a common center of mass located between the two bodies (rather than within one of the bodies).^{[note 1][16]} The IAU currently states that Charon is not considered to be a dwarf planet and is just a satellite of Pluto, although the idea that Charon might qualify to be a dwarf planet in its own right may be considered at a later date.^[68] The location of the barycenter depends not only on the relative masses of the bodies, but also on the distance between them; the barycenter of the Sun–Jupiter orbit, for example, lies outside the Sun.

Eris (dwarf planet)

From Wikipedia, the free encyclopedia

Eris

Eris (center) and Dysnomia (left of center), taken by the Hubble Space Telescope
Discovery
Discovered by	M. E. Brown C. A. Trujillo D. L. Rabinowitz[1]
Discovery date	January 5, 2005[2][a]
Designations
MPC designation	(136199) Eris
Pronunciation	/ˈɪərɪs/ or /ˈɛrɪs/[b]
Named after	Eris
Alternative designations	2003 UB313[3]
Minor planet category	Dwarf planet TNO Plutoid SDO Binary[4][5]
Adjectives	Eridian
Orbital characteristics[3]
Epoch December 9, 2014 (JD 2457000.5)
Earliest precovery date	September 3, 1954
Aphelion	97.651 AU 14.602×109 km
Perihelion	37.911 AU 5.723×109 km
Semi-major axis	67.781 AU 10.166×109 km
Eccentricity	0.44068
Orbital period	203,830 d 558.04 yr
Average orbital speed	3.4338 km/s
Mean anomaly	204.16°
Inclination	44.0445°
Longitude of ascending node	35.9531°
Argument of perihelion	150.977°
Known satellites	Dysnomia
Physical characteristics
Dimensions	2326±12 km
Mean radius	1163±6 km[8][9]
Surface area	(1.70±0.02)×107 km2[c]
Volume	(6.59±0.10)×109 km3[c]
Mass	(1.66±0.02)×1022 kg[10] 0.0028 Earths 0.23 Moons
Mean density	2.52±0.07 g/cm3[d]
Equatorial surface gravity	0.82±0.02 m/s2 0.083±0.002 g[e]
Equatorial escape velocity	1.38±0.01 km/s[e]
Sidereal rotation period	25.9±0.5 hr[11]
Geometric albedo	0.96+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]
Apparent magnitude	18.7[13]
Absolute magnitude (H)	−1.17+0.06 −0.11[f]
Angular diameter	40 milli-arcsec[15]

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 identity 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.^[8] Eris's 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 most distant known natural objects in the Solar System.^[18][g]

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,^[21] 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,^[22][23] which was measured by New Horizons as 2,372 ± 4 kilometers (1,473.9 ± 2.5 mi) in July 2015.^[24]

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,^[25] 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]

Precovery images of Eris have been identified back to September 3, 1954.^[3]

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.^[26] The name was proposed by the CalTech Team on September 6, 2006, and it was assigned on September 13, 2006,^[27] 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 and its large moon Dysnomia. 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 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.^[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 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.^[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 24 August 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

Seen from earth, Eris makes small loops in the sky through the constellation of Cetus

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^[5] 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 most distant known objects in the Solar System, 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.^[4]

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.

Size, mass and density

Size estimates

Year	Radius (diameter)	Source
2005	1,199 (2,397) km[50]	Hubble
2007	1,300 (2,600) km[51]	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).^[50][52] 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,^[j] sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.^[k]

Artistic comparison of Pluto, Eris, Makemake, Haumea, Sedna, 2002 MS₄, 2007 OR₁₀, Quaoar, Salacia, Orcus, and Earth along with the Moon.

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.^[54]

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.^[51] 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][55] 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.^[56]

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.^[57]

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.^[58] 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.^[59]

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.^[60]

Satellite

In 2005, the adaptive optics team at the Keck telescopes in Hawaii carried out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris), using the newly commissioned laser guide star adaptive optics system.^[61] 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.^[62]

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.^[63]