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A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body in direct orbit of the Sun that is massive enough that its shape is controlled by gravitational forces rather than mechanical forces (and is thus an ellipsoid), but has not cleared the neighboring region of other objects. More explicitly, it is a planetary-mass object—it has sufficient mass to overcome its internal compressive strength and achieve hydrostatic equilibrium—but is neither a planet nor a satellite.
The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between. 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 Mike Brown, who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern, who had coined the term dwarf planet in 1990.
The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake, and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. The IAU 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) are to be named under the assumption that they are dwarf planets. The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.
It is suspected that about another hundred known objects in the Solar System are dwarf planets. Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number might be as high as 2,000 when objects scattered outside the Kuiper belt are considered. Mike Brown published in August 2011 a list of 390 candidate objects, organized in categories from "nearly certainly" to "possibly" meeting the IAU's criteria, along with his classification methodology. Brown identifies nine known objects – the five mentioned plus 2007 OR10, Sedna, Quaoar, and Orcus – as "virtually certain", with another two dozen highly likely, and there are probably a hundred or so such objects in total.
Before the discoveries of the early 21st century, astronomers had no strong need for a formal definition of a planet. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies such as asteroids and comets. For almost 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and determine that it is much smaller than the initial estimates. 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 was one-fifth that of Earth's Moon. Furthermore, having some unusual characteristics such as large orbital eccentricity and a high orbital inclination, it became evident it was a completely different kind of body from any of the other planets.
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. Many of these shared some of the key orbital characteristics of Pluto, 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 minor planet, subplanet, and planetoid started to be used for the bodies now known as dwarf planets. By 2005, three other bodies comparable to Pluto in terms of size and orbit (Quaoar, Sedna, and Eris) had been reported in the scientific literature. It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified. 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.
In 2006, Eris (then known as 2003 UB313) was believed to be slightly larger than Pluto, and some reports unofficially referred to it as the tenth planet. As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006. 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, in which he created a median classification for objects large enough to be round but that had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, since they have not cleared their orbits.
The IAU's final resolution preserved this three-category system for the celestial bodies orbiting the Sun. Fernández suggested calling these median objects planetoids, but the IAU's division III plenary session voted unanimously to call them dwarf planets. The resolution, #5A, 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."
- 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.
The term dwarf planet has itself been somewhat controversial, as a grammatical reading suggests these bodies are planets. The IAU's Resolution 5A was accompanied by a second resolution, 5B, which defined dwarf planets as a subtype of planet, distinguished from the other eight which 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". However, resolution 5B was defeated in the same session that 5A was passed, so that only the dwarf planet half of the proposal was made official. Because of the grammatical inconsistency of a dwarf planet not being a planet, while a dwarf star is a star, alternative proposals such as nanoplanet and subplanet were suggested. However, it was judged that the term dwarf planet had already entered general use and that it was too late to change it. The term is parallel to minor planet, which is also not a type of planet.
In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French planète naine, German Zwergplanet, Russian карликовая планета karlikovaya planeta, Arabic كوكب قزم kaukab qazm, Chinese 矮行星 ǎixíngxīng, etc., but Japanese is an exception: In Japanese these bodies are called junwakusei 準惑星, where wakusei 惑星 is 'planet' and jun- 準 is a prefix corresponding to English quasi-, pene- (almost), and sub-. Thus in Japanese they are called 'subplanets' or 'almost-planets'. The modern Latin name for this class of objects, planetulus, or planetion following a Greek-styled name, is also a diminutive form of planeta, hence meaning "small planet".
Although concerns were raised about the classification of planets orbiting other stars, the issue was not resolved; it was proposed instead to decide this only when such objects start being observed.
The 2006 IAU's Resolution 6a 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. On June 11, 2008, the IAU Executive Committee announced a name, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids, though "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." On July 18, 2008, the WG-PSN classified the object then known as (136472) 2005 FY9 as a dwarf planet, and named it Makemake; this was followed in September by Haumea.
|Mercury||0.055||1.95 × 103||9.1 × 104|
|Venus||0.815||1.66 × 105||1.35 × 106|
|Earth||1||1.53 × 105||1.7 × 106|
|Mars||0.107||9.42 × 102||1.8 × 105|
|Ceres||0.000 15||8.32 × 10−4||0.33|
|Jupiter||317.7||1.30 × 109||6.25 × 105|
|Saturn||95.2||4.68 × 107||1.9 × 105|
|Uranus||14.5||3.85 × 105||2.9 × 104|
|Neptune||17.1||2.73 × 105||2.4 × 104|
|Pluto||0.002 2||2.95 × 10−3||0.077|
|Haumea||0.000 67||2.68 × 10–4||0.02|
|Makemake||0.000 67||2.22 × 10–4||0.02|
|Eris||0.002 8||2.13 × 10−3||0.10|
*ME in Earth masses.
**Λ = k M2 a−3/2,
where k = 0.0043 for units of Yg and AU. Λ > 1 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. µ > 100 for planets.
Alan Stern and Harold F. Levison introduced a parameter Λ (lambda), expressing the likelihood of an encounter resulting in a given deflection of orbit. The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. Following the authors, 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.
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), while dwarf planets lack the mass to do so. 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. There are several other schemes that try to differentiate between planets and dwarf planets, but the 2006 definition uses this concept.
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 be angular in 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 it achieves maximum rounding at hydrostatic equilibrium. This is the defining limit of a dwarf planet.
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 does rotate, the more oblate or even scalene it becomes. However, 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.
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. 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", but this was not retained in the final draft.
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 some 3.4×1020 kg. For a body made of less rigid water ice, the limit should be about 320 km and ×1019 kg. 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, however, 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.
Among icy bodies, the smallest known to be in hydrostatic equilibrium is Mimas, at 396 km and 3.75×1019 kg. The largest irregular body in the outer Solar System is Proteus, nearly-but-not-quite round at 405–435 km and an assumed mass of ≈4.4×1019 kg. Bodies like Mimas may have had a warmer thermal history than Proteus, or their shape may have resolved after a collision. Neither body is pure ice as used to calculate the lowest limit, however, and Mike Brown suggests that the practical lower limit for an icy dwarf planet is likely to be somewhere under 400 km. There are about 100 TNOs currently estimated to be above this size.
As of 2011, five objects are recognized as dwarf planets by the IAU, with four others also thought to be "nearly certain". For two of these, Ceres and Pluto, this is known through direct observation. The other seven are thought to be massive enough to be in hydrostatic equilibrium even if they are dense (primarily rocky) and at the lower end of their estimated diameters. Eris is more massive than Pluto; Haumea and Makemake were accepted as dwarf planets based on their absolute magnitudes. In relative distance from the Sun, the five are:
Brown identifies four other objects that "must be dwarf planets even if predominantly rocky":
No space probes have visited any of these. This will change if NASA's Dawn and New Horizons missions reach Ceres and Pluto, respectively. The plans are for Dawn to orbit Ceres, and New Horizons to fly by Pluto, both in 2015. Dawn entered orbit around the potential dwarf planet Vesta on July 16, 2011.
|Orbital attributes of recognized dwarf planets|
|Pluto||Kuiper belt (plutino)||39.48||248.09||4.666||17.14°||0.249||0.077|
|Haumea||Kuiper belt (12:7)||43.13||283.28||28.22°||0.195||0.020|
|Makemake||Kuiper belt (cubewano)||45.79||309.9||4.419||28.96°||0.159||0.02|
|Physical attributes of recognized dwarf planets|
|Haumea||≈ 37%||1300±?||5.5%||4.01 ± 0.04||2.6–3.3 (?)||0.44||0.84||0.16||2||32 ± 3||?|
|Makemake||41%||1420±60||≈ 4% ?||≈ 3 ?||?||?||0.32||0||≈ 30||transient?|
|Eris||67%||2326±12||22.7%||16.7||2.5||≈ 0.8||1.3||≈ 1 (0.75–1.4)||1||≈ 42||transient?|
|Orbital attributes of "nearly certain" dwarf planets|
|Orcus||Kuiper belt (plutino)||39.17||245.18||20.57°||0.227||0.003|
|Quaoar||Kuiper belt (cubewano)||43.405||285.97||8.00°||0.039||0.007–0.010|
|2007 OR10||Scattered disc (10:3?)||67.21||550.98||30.70°||0.500||?|
|Physical attributes of "nearly certain" dwarf planets|
|Quaoar||≈ 26%||890±70||1.8–2.6%||1.6 ± 0.3||°||0.74||1|
|2007 OR10||≈ 37%||1280±210||?||?||?||0|
|Sedna||≈ 30%||995 ± 80||≈1.4%||≈1||?||0.42||0||≈ 12|
After Ceres, the next-most-massive body in the asteroid belt, Vesta, might also be classified as a dwarf planet, as its shape appears to deviate from hydrostatic equilibrium mainly because of a large impact that occurred after it solidified. The definition of dwarf planet does not address this issue. The Dawn probe orbiting Vesta since July 2011 may help clarify matters.
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, making them dwarf planets of the plutoid class. Although only rough estimates of the diameters of these objects are available, as of 2011 it was believed that a hundred of the known bodies beyond Neptune were probable dwarf planets. A team is investigating thirty of these, and believe that the number will eventually prove to be about 200 in the Kuiper belt and many more beyond.
Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity. These bodies have no significant physical differences from the dwarf planets, but are not members of that class under the IAU definition because they do not directly orbit the Sun. They are Earth's moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), seven moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), one moon of Neptune (Triton), and one moon of Pluto (Charon). The term planemo ("planetary-mass object") covers both dwarf planets and such moons, as well as planets. Alan Stern considers them a special category of planets, "satellite planets".
Despite its requirement that dwarf planets orbit the Sun directly, draft resolution (5) presented to the IAU, which considered dwarf planets to be a category of planet, stated Charon could be considered a planet because it revolves with Pluto around a common center of mass located between the two bodies (rather than within one of the bodies).[note 1] Note, however, that the Jupiter–Sun centre of mass lies outside the Sun, and so such a definition would imply that Jupiter is not a satellite of the Sun and therefore not a planet. This definition was not preserved in the IAU's final resolution.
In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution. Campaigns included car bumper stickers and T-shirts. Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.
NASA has announced that it will use the new guidelines established by the IAU. However, 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. Thus, as of January 2008, he and his team still referred to Pluto as the ninth planet, and accept other dwarf planets such as Ceres and Eris as additional planets. Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both 'planets'.
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