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A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite.[1][2] More explicitly, it has to have sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium. It should not be confused with a minor planet.

The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,[3] 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 larger object, Eris.[4] 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 definition officially adopted by the IAU in 2006 has been both praised and criticized, and remains disputed by some scientists.

The IAU currently recognizes five dwarf planets—Ceres, Pluto, Haumea, Makemake, and Eris.[5] 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 less than +1 (and hence a mathematically delimited minimum diameter of 838 km[6]) 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 at least another 40 known objects in the Solar System are dwarf planets,[7] and 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.[7] The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed,[8] although if they were detectable they would not be considered planets.[9]
Contents
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* 1 History of the concept
* 2 Characteristics
o 2.1 Orbital dominance
o 2.2 Size and mass
* 3 Current members
* 4 Candidates
o 4.1 Plutoid candidates
o 4.2 Ellipsoidal moons
* 5 Contention
* 6 See also
* 7 References
* 8 External links

History of the concept
Main article: 2006 definition of planet

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,[10][11] but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure the mass of Pluto accurately and it was noticed that actual mass was much smaller than the initial estimates.[12] 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.[13] 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.[14]

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.[15] 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.[16][17] 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.[18] It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified.[19] 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.[20]

In 2006, Eris (then known as 2003 UB313) was determined to be slightly larger than Pluto, and some reports unofficially referred to it as the tenth planet.[21] As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006.[22] 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.[23]

The IAU's final resolution preserved this three-category system for the celestial bodies orbiting the Sun. Fernández suggested calling these median objects planetoids,[24][25] but the IAU's division III plenary session voted unanimously to call them dwarf planets.[3] The resolution read, in full:
“ 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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3, 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 there were concerns about the classification of planets in other solar systems,[8] this issue was not resolved; it was proposed instead to decide this only when such objects will start being observed.[23]

The 2006 IAU's Resolution 6a[26] 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.[3] On June 11, 2008, the IAU Executive Committee announced a name, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids.[27] On July 11, 2008, the Working Group for Planetary System Nomenclature reclassified the object then known as (136472) 2005 FY9 as a dwarf planet, and renamed it Makemake.[5]

Characteristics
Planetary discriminants[28] Body ↓ Mass (ME*)
↓ Λ/ΛE**
↓ µ*** ↓
Mercury 0.055 0.012 6 9.1 × 104
Venus 0.815 1.08 1.35 × 106
Earth 1.00 1.00 1.7 × 106
Mars 0.107 0.006 1 1.8 × 105
Ceres 0.000 15 8.7 × 10−9 0.33
Jupiter 317.7 8 510 6.25 × 105
Saturn 95.2 308 1.9 × 105
Uranus 14.5 2.51 2.9 × 104
Neptune 17.1 1.79 2.4 × 104
Pluto 0.002 2 1.95 × 10−8 0.077
Haumea 0.000 67 1.72 × 10–9 0.02
Makemake 0.000 67 1.45 × 10–9 0.02[29]
Eris 0.002 8 3.5 × 10−8 0.10

*ME in Earth masses.
**Λ/ΛE = M²/P × PE/M2E.
***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.

Orbital dominance
Main article: Clearing the neighbourhood

Alan Stern and Harold F. Levison introduced a parameter Λ (lambda), expressing the probability of an encounter resulting in a given deflection of orbit.[30] 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. A gap of five orders of magnitude in Λ was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects (third column of the planetary discriminants table to the right).[28]

Using this parameter, Steven Soter and other astronomers argued for a distinction between dwarf planets and the other eight planets based on their inability to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance, while dwarf planets lack the mass to do so.[30] In other words, 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).[28] There are several other schemes that try to differentiate between planets and dwarf planets,[30] but the 2006 definition uses this concept.[3]

Size and mass
Main article: Hydrostatic equilibrium

When an object achieves hydrostatic equilibrium, also known as gravitational relaxation, there are no gravitational imbalances in its surface. A global layer of liquid placed on this surface (assuming for argument's sake it would remain a liquid) would form a liquid surface of the same shape, apart from small-scale surface features such as craters and fissures. This does not mean the body is a sphere; the faster a body rotates, the more oblate or even scalene it becomes, but such forces affect a liquid surface as well. 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 relative masses of the five known dwarf planets, plus Charon. The mass of Makemake is a rough estimate.
Relative masses including the Earth's Moon.

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.[31] 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",[8] but this was not retained in the final draft.[3]

Empirical observations suggest that the lower limit may vary according to the composition of the object. For example, in the asteroid belt, Ceres, with a diameter of 975 km, is the only object known to presently be self-rounded (though Vesta may once have been). Therefore, it has been suggested that the limit where other rocky-ice bodies like Ceres become rounded might be somewhere around 900 km.[7] More icy bodies like trans-Neptunian objects have less rigid interiors and therefore more easily relax under their self-gravity into a rounded shape.[7] The smallest icy body known to have achieved hydrostatic equilibrium is Mimas, while the largest irregular one is Proteus; both average slightly more than 400 km (250 mi) in diameter. Mike Brown (a leading researcher in this field and discoverer of Eris) suggests that the lower limit for an icy dwarf planet is therefore likely to be somewhere under 400 km.[7]

It is also not clear to what extent deviations from perfect equilibrium are to be tolerated, or whether having achieved equilibrium is sufficient for inclusion. All solid bodies in the solar system, such as Iapetus with its equatorial ridge and Mars with its shield volcanoes, deviate to some extent. This may be a critical for the consideration of the asteroid 4 Vesta, which may deviate from equilibrium due to a large impact that removed part of one hemisphere.

Current members
Haumea with its moons, Hiʻiaka and Namaka (artist's conception)
Makemake (artist's conception)
Eris (through the Hubble Space Telescope)
Ceres (through the Hubble Space Telescope)
Pluto (approximate true color)

As of 2008, the IAU has classified five celestial bodies as dwarf planets. Two of these, Ceres and Pluto, are known to qualify as dwarf planets through direct observation. The other three, Eris, Haumea, and Makemake, are thought to be dwarf planets from mathematical modeling—or in the case of Eris, because it is larger than Pluto—and qualify for the classification under IAU naming rules based on their magnitudes.[5][26]

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

No space probes have visited any of the dwarf planets. This will change if NASA's Dawn and New Horizons missions reach Ceres and Pluto, respectively, as planned in 2015.[32][33] Dawn is also slated to orbit and observe another potential dwarf planet, Vesta, in 2011.
Orbital attributes of dwarf planets[34]
Name Region of
Solar System Orbital
radius (AU) Orbital period
(years) Mean orbital
speed (km/s) Inclination
to ecliptic (°) Orbital
eccentricity Planetary
discriminant
Ceres Asteroid belt 2.77 4.60 17.882 10.59 0.080 0.33
Pluto Kuiper belt 39.48 248.09 4.666 17.14 0.249 0.077
Haumea Kuiper belt 43.34 285.4 4.484 28.19 0.189 ?
Makemake Kuiper belt 45.79 309.9 4.419 28.96 0.159 ?
Eris Scattered disc 67.67 557 3.436 44.19 0.442 0.10
Physical attributes of dwarf planets
Name Equatorial
diameter
relative to
the Moon Equatorial
diameter
(km) Mass
relative to
the Moon Mass
( × 1021 kg) Density
( × 103g/m³) Surface
gravity
(m/s2) Escape
velocity
(km/s) Axial
inclination Rotation
period
(days) Moons Surface
temp.
(K) Atmosphere
Ceres[35][36] 28.0% 974.6±3.2 1.3% 0.95 2.08 0.27 0.51 ~3° 0.38 0 167 none
Pluto[37][38] 68.7% 2306±30 17.8% 13.05 2.0 0.58 1.2 119.59° -6.39 3 44 transient
Haumea[39][40] 33.1% 1150+250−100 5.7% 4.2 ± 0.1 2.6–3.3 ~0.44 ~0.84 2 32 ± 3 ?
Makemake[39][41] 43.2% 1500+400−200 ~5%? ~4? ~2? ~0.5 ~0.8 0 ~30 transient?
Eris[42][43] 74.8% 2400±100 22.7% 16.7 2.3 ~0.8 1.3 ~0.3 1 42 transient?

Candidates

As with Ceres, the next three largest objects in the main asteroid belt – Vesta, Pallas, and Hygiea[44] – could eventually be classified as dwarf planets if it is shown that their shape is determined by hydrostatic equilibrium.[45] While uncertain, the present data suggests that it is unlikely for Pallas and Hygiea. Vesta, however, appears to deviate from hydrostatic equilibrium only because of a large impact that occurred after it solidified;[46] the definition of dwarf planet does not specifically address this issue. The Dawn probe scheduled to enter orbit around Vesta in 2011 may help clarify matters.[32]

The status of Charon (currently regarded as a satellite of Pluto) remains uncertain, as there is currently no clear definition of what distinguishes a satellite system from a binary (double planet) system. The original draft resolution (5)[8] presented to the IAU stated that Charon could be considered a planet because:

1. Charon independently would satisfy the size and shape criteria for a dwarf planet status (in the terms of the final resolution);
2. Charon revolves with Pluto around a common barycentre located between the two bodies (rather than within one of the bodies) because Charon's mass is not insignificant relative to that of Pluto.[47]

This definition, however, was not preserved in the IAU's final resolution and it is unknown if it will be included in future debates.

Plutoid candidates
See also: List of plutoid candidates
Illustration of the relative sizes, albedos, and colours of the largest Trans-Neptunian objects

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.[7] Although only rough estimates of the diameters of these objects are available, as of August 2006, it was believed that another 42 bodies beyond Neptune (besides Pluto and Eris) were likely dwarf planets.[7][48] A team is investigating another 30 such objects, and believe that the total number will eventually prove to be about 200 in the Kuiper belt, and many more beyond it.[7]

Tancredi & Favre (2008) attempt to estimate which TNOs are likely to qualify, based on both direct measurements and lightcurve data. They propose that nine of the candidates be considered dwarf planets.[49] Six of these have been estimated by one researcher or another to be at least 900 km in diameter, the size of the smallest known dwarf planet, Ceres, as has a tenth candidate, 2002 AW197. These ten prime candidates are:
Prime plutoid candidates[50] Name Category Estimated diameter (km) Magnitude
(H) Mass
( × 1020 kg) Orbital
radius
(AU)
by [7] by [51] by [52] by [53]
Orcus plutino
(1 moon) 1,100 909 946 1,500 6.2–7.0 39.12
Pluto 39.48
Ixion plutino 980 570 650 1,065 ~5.8 39.65
Huya plutino 480 480 — — 0.8–1.6? 39.76
Varuna cubewano 780 874 500 900 ~5.9 42.90
2002 TX300 Haumean
cubewano 800 709 — — 1.6–3.7 43.11
Haumea 43.34
Quaoar cubewano
(1 moon) 1,290 1,260 844 1,200 10–26 43.58
Makemake 45.79
2002 AW197 cubewano 940 793 735 890 ~5.2 47.30
2002 TC302 5:2 SDO 710 1,200 1,150 — 0.78 55.02
Eris 67.67
1996 TL66 SDO — 632 460–690 — 2.6? 82.90
Sedna detached object 1,800 1,500 < 1,600 < 1,500 17–61 486.0

Additionally, more-recently discovered 2007 OR10 should probably be seen as a prime candidate, as Mike Brown estimates its size to be between that of Sedna and Quaoar[54].

Ellipsoidal moons
See also: List of moons by diameter

A total of 19 known moons are 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 considered members of that class 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).

Contention

In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution.[55] Campaigns included car bumper stickers and T-shirts.[56] Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.[57]

NASA has announced that it will use the new guidelines established by the IAU.[58] However, Alan Stern, the director of the 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.[59] Thus, as of January 2008, he and his team still refer to Pluto as the ninth planet,[60] while accepting the characterization of dwarf planet for Ceres and Eris.[citation needed]