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File:Pluto artistimpression.gif

Artist's impression of Pluto (background) and Charon (foreground). Pluto, considered a planet for 76 years, was reclassified as a dwarf planet in 2006.

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 which has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass for its self-gravity to overcome rigid body forces in order to assume a hydrostatic equilibrium and acquire a near-spherical shape. . Dwarf planets are a subset within the category of minor planets.

The term dwarf planet was adopted in 2006 as part of a three-way classification of bodies orbiting the Sun. This classification states that bodies that are large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to have to have acquired a round shape are defined as small solar system bodies. Dwarf planets come in between, and have been called by some astronomers as "something that looks like a planet, but is not a planet". Even now there is no uncontested distinction between the term planet and dwarf planet and the definition officially adopted by the IAU in 2006 has been both praised and criticized, and remains disputed by some scientists.

While the upper limit for the size of dwarf planets has not been addressed at all, the lower limit has been an issue of debate. It is thought that warmer bodies that have a rocky core require a diameter of around 900 km (similar to that of Ceres) to be spherical, while colder bodies that have an icy core could require only a diameter of 400km. Little is known about the other characteristics of dwarf planets, but this will presumably change if NASA spacecraft reach Ceres and Pluto in 2015.

The IAU currently recognizes only three dwarf planets - Ceres, Pluto, and Eris - but more than 70 other objects in the Solar System are suspected to be candidates for inclusion in this category. There are estimations that the number might increase to 200 when the region known as Kuiper belt will be fully explored, and might be around 2000 when also objects outside this region will be accounted for. The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed, though if they were detectable they would not be considered planets.

History of the name

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 actually thought to be larger than Mercury. However, 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 estimations. It was roughly one twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still over ten times as massive as the largest object in the asteroid belt, Ceres, it was five times even less than that of Earth's Moon. Furthermore, having some unusual characteristics such as large orbital eccentricity and a very inclined orbit, it became evident that it was a very different body from any of the other planets.

In the 1990s, astronomers began to find objects in the same region of space (now known as Kuiper Belt) as Pluto, and some even further away. Many of these shared some key orbital characteristics as Pluto, and Pluto started being seen as the largest member of a new class of objects, plutinos. This lead to 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 a dwarf planets. By 2005, three other bodies (Quaoar, Sedna and Eris) comparable to Pluto in terms of size and orbit had been reported in the scientific literature. It became clear that they all would either have to be called planets too, or Pluto would have to be reclassified. Astronomers were also very 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, the size of 2003 UB₃₁₃ (named later Eris) was measured to be slightly larger than Pluto's. It started being referred to as the 10th planet since it was thought to be equally deserving of the status of 'planet' as Pluto. As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006. A final resulotion was passed in favor of a three-category system for the celestial bodies orbiting the Sun:

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 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, 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Footnotes:

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

An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

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

Althouigh there were concerns about the classification of planets in other solar systems, the issue was not resolved.

Characteristics

Planetary discriminants
Body	Mass (M_E)	Λ/Λ_E	µ
Mercury	0.055	0.0126	9.1×10
Venus	0.815	1.08	1.35×10
Earth	1.00	1.00	1.7×10
Mars	0.107	0.0061	1.8×10
Ceres	0.00015	8.7×10	0.33
Jupiter	317.7	8510	6.25×10
Saturn	95.2	308	1.9×10
Uranus	14.5	2.51	2.9×10
Neptune	17.1	1.79	2.4×10
Pluto	0.0022	1.95×10	0.077
Eris	0.0028	3.5×10	0.10

*M_E in Earth masses.
**Λ/Λ_E = M²/P, in Earth masses squared per year.
***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: Cleared the neighbourhood

Using a parameter developed by S. Alan Stern and Harold F. Levison, Steven Soter and other astronomers have argued for a distinction between dwarf planets and the other eight planets based on their inability to "clear the neighborhood around their orbits". In other words, they need to remove smaller bodies whose orbits bring them nearby by collision, capture, or gravitational disturbance. 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). Dwarf planets have too little mass to significantly alter their environment the way a planet does. There are several other theories that try to differentiate between planets and dwarf planets, but the current definition of what constitutes a planet uses this concept.

Stern et al. introduced a parameter Λ (lambda), expressing the probability 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. A gap of five orders of magnitude in Λ was found between the smallest terrestrial planets and the largest asteroids and KBOs (second column in the nearby table).

Size and mass

The upper and lower limits to the size and the mass of dwarf planets have not been specified by the IAU. There is strictly no upper limit, and an object larger or more massive than Mercury that would be considered not to "have cleared the neighborhood around its orbit" may be classified as a dwarf planet. The lower limit is determined by the concept of hydrostatic equilibrium shape, but the size or mass at which an object attains this shape has not been defined. The original draft of the 2006 IAU resolution defined hydrostatic equilibrium shape as applying "to objects with mass above 5×10 kg and diameter greater than 800 km", but this was not retained in the final draft.

Empirical observations suggest that the lower limit may vary according to the composition and history of the object. For example, in the asteroid belt, Ceres, with a diameter of 900 km, is the only object known for sure to be round. Therefore, somewhere around 900 km would be good cutoff for rocky bodies like asteroids. Icy bodies like TNOs (much colder and with a lot of ice in their interiors) have less rigid interiors and therfore it is easier for the gravity to give them a spherical shape - similarly to how "it takes less force to make an ice ball round". The smallest icy body known to have achieved hydrostatic equilibrium is Mimas, while the largest irregular one is Proteus, and both average slightly more than 400 km 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. According to these guidelines, there were a total of 45 known bodies in August 2006 that could qualify as dwarf planets, but now the number has increased to around 80.

Current members

The IAU has classified three celestial bodies as dwarf planets:

Ceres - discovered on January 1, 1801 (16 years before Neptune), considered a planet for half a century before reclassification as an asteroid;
Pluto - discovered on February 18, 1930, classified as a planet for 76 years;
Eris - discovered on October 21, 2003

Here is a comparison of the main characteristics of these three bodies:

Physical attributes of dwarf planets
Name	Relative equatorial diameter	Equitorial diameter (km)	Relative mass	Mass (×10 kg)	Density (×10g/m³)	Surface gravity (m/s)	Escape velocity (km/s)	Axial inclination	Rotation period (days)	Moons	Surface temp. (K)	Atmosphere
Ceres	0.074	974.6±3.2	0.0002	0.095	2.08	0.27	0.51	~3°	0.38	0	167	none
Pluto	0.19	2306±30	0.0021	1.305	2.0	0.58	1.2	119.59°	-6.39	3	44	temporary
Eris	0.19	2400±100	0.0025	1.67	2.3	~0.8	1.3		~0.3	1	42	temporary
Measured relative to the Earth.

Orbital attributes of dwarf planets
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
Eris	Scattered disc	67.67	557	3.436	44.19	0.442	0.10

Candidates

As with Ceres, the next three largest objects in the main asteroid belt - Vesta, Pallas and Hygiea - could also end up classified as dwarf planets if it is shown that their shape is determined by hydrostatic equilibrium. 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 Vesta solidified (the definition of dwarf planet does not specifically address this issue). The Dawn probe that is scheduled to enter orbit around Vesta in 2011 may help clarify whether Vesta matches the definition of a dwarf planet.

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

Charon independently would satisfy the size and shape criteria for a dwarf planet status (in the terms of the final resolution);
Charon, since it has mass large enough relative to Pluto, revolves with Pluto around a common barycentre located outside the surface of Pluto (rather than around a point located within).

This definition, however, was not preserved in the IAU's final resolution and it is unknown if it will be included in future debates. If indeed such a definition were to be adopted, the Pluto-Charon system would then be considered a double dwarf planet.

Most likely TNO candidates

Trans-Neptunian objects (or simply TNOs) are thought to have icy cores and therefore would require a diameter of probably less than 400 km. Although it is difficult to measure the diameters of these objects, Brown believes that (as of August 2006) in addition to Pluto and Eris, 42 other bodies in the Kuiper belt and the Sedna (in the Scattered disc), met this requirement and qualified as dwarf planets. His team is investigating another 30 such objects that they have yet to announce, and believe that the total number will eventually prove to be about 200 in the Kuiper belt, with many more beyond it.

It is debatable weather most of these candidates will be proven to have definately reached a spherical shape - and therefore be classified as a dwarf planet by IAU. The bodies that have a large enough size, and especially those that have a diameter larger than Ceres' (around 900km) are more likely to be spherical and will probably listed by IAU once their shape is revealed by accurate analysis. The TNOs that are known to have such diameters are either cubewanos, plutinos, or scattered disc objects (SDOs). Hereby is a list of them:

Name	Category	Diameter (km)				Mass (×10 kg)	Orbital radius (AU)
Name	Category	by	by	by	others	Mass (×10 kg)	Orbital radius (AU)
2003 EL₆₁	cubewano	2000	1380	1350		~42	43.31
Sedna	extended-SDO	1800	1500	<1800		17-61	486.0
2005 FY₉	cubewano	1600	1500	1500		~40	45.66
Quaoar	cubewano	1290	1260	1260		10-26	43.58
Orcus	plutino	1100	909	946		6.2 - 7.0	39.34
Ixion	plutino	980	570	650	900-1230	~5.8	39.65
Ceres	-	975				9.5	2.77
2002 AW197	cubewano	940	793	977	770-1010	~5.2	47.30
Varuna	cubewano	780	874	1016	755-1025	~5.9	42.90
2002 TC302	SDO	710	1200	1150		0.78	55.02

Pluto prototype

Main article: Pluto prototype

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 are not specified, but in the debate leading up to the resolution, the members of the category were variously referred to as Plutons and Plutonian objects. The former name was generally deprecated and was abandoned in the final draft resolution (6b); the latter name failed to win majority approval on a 180–186 vote in the IAU General Assembly on August 24 2006. The category, while established, remains nameless.

At an earlier stage in the definition process, the category (then described as "pluton") was defined to be a planet whose orbit took more than 200 Julian years to complete and whose orbit was more highly inclined and elliptical than a traditional planetary orbit.

This category of Pluto-like objects only applies to dwarf planets which meet the conditions of being trans-Neptunian and "like Pluto" in terms of period, inclination and eccentricity. A dwarf planet may or may not be a member of this category, but all members of the category must be dwarf planets.

The membership of this class, other than Pluto itself, remains obscure. Eris and the objects listed in the table above do qualify in terms of the minimum period, and most exhibit orbital eccentricity and inclination that are significant (though not always equal to or greater than Pluto's). Quaoar, however, has a much smaller eccentricity and inclination, and so possibly does not qualify as a Pluto-like object.

Contention

A number of scientists expressed their disagreement with the currently adopted IAU definition of dwarf planet by means of car bumper stickers and T-shirts. However, Mike Brown (the discoverer of the would-have-been-the-10th-planet Eris) agrees with the reduction of the number of planets to eight.

While accepting the characterisation of dwarf planet for Ceres and Eris (dwarf planet in this case meaning just a small planet), the director of the NASA's mission to Pluto, Alan Stern, 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, he and his team will still refer to Pluto as the ninth planet. However, NASA has announced that it will use the new guidelines established by the IAU.

References

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^ Michael E. Brown. "The Dwarf Planets". Retrieved 2008-01-20.
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^ Stern, S. Alan (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (PDF). Highlights of Astronomy. 12: 205–213, as presented at the XXIVth General Assembly of the IAU - 2000 Manchester, UK, 7-18 August 2000. Retrieved 2008-01-22. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
Soter, Steven (2006-08-16). "What is a Planet?" (PDF). The Astronomical Journal. 132: 2513–2519. Retrieved 2008-01-22.
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External links

NPR: Dwarf Planets May Finally Get Respect (David Kestenbaum)
BBC News: Q&A New planets proposal Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
Ottawa Citizen: The case against Pluto (P. Surdas Mohit) Thursday, August 24 2006

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