Dwarf Planet

Information about Dwarf Planet

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 is a celestial body within the Solar System that satisfies the following four conditions:^[1]

is in orbit around the Sun
has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (near-spherical) shape
has not cleared the neighbourhood around its orbit
is not a satellite

The term dwarf planet was adopted in 2006 as part of a three-way classification of bodies orbiting the Sun. Objects that are large enough to have cleared the neighbourhood of their orbit are defined as planets, while those which are too small to be in hydrostatic equilibrium are defined as small solar system bodies. The category dwarf planet is not a subset of the category planet, but a separate category altogether; that is to say, a dwarf planet is not a planet. As defined, the term dwarf planet does not apply to other planetary systems.^[2]

Three dwarf planets are currently recognized: Ceres, Pluto and Eris.

List of dwarf planets

The IAU has officially identified three celestial bodies that have immediately received dwarf planet classification:^[3]

**Dwarf planets**
Name	Ceres	Pluto	Eris
MPC number	1	134340	136199
Region of Solar System	Asteroid belt	Kuiper belt	Scattered disc
Diameter	941±32 km	2306±30 km	2400±100 km
Mass in kg compared to Earth	9.510²⁰ kg 0.00016	1.30510²² kg 0.0022	~1.6710²² kg^[4] 0.0028
Mean equatorial radius* in km	0.0738 471	0.180 1,148.07	0.19 ~1,200
Volume*	0.00042	0.005	0.007
Density (in Mg/m³)	2.08	2.0	2.1
Equatorial gravity (in m/s²)	0.27	0.60	~0.68
Escape velocity (in km/s)	0.51	1.2	~1.3
Rotation period (d) (in sidereal days)	0.3781	-6.38718 (retrograde)
Orbital radius^* (AU) semi-major axis in km	2.5-2.9 2.766 413,715,000	29.66-49.30 39.48168677 5,906,376,200	37.77-97.56 67.6681 10,210,000,000
Orbital period^*(a) (in sidereal years)	4.599	248.09	557
Mean orbital speed (in km/s)	17.882	4.666	3.437
Orbital eccentricity	0.080	0.24880766	0.44177
Orbital inclination	10.587°	17.14175°	44.187°
Inclination of the equator from the orbit (see Axial tilt)	4°	119.61°
Mean surface temperature (in K)	167	40	30
Number of natural satellites	0	3	1
Date of discovery	January 1, 1801	February 18, 1930	October 21, 2003

*Measured relative to the Earth.

Other candidates

Additionally, there are several bodies potentially qualifying as dwarf planets. Among these, the following are known or thought to be greater than around 700 km in diameter:

**Possible dwarf planets**
Name	Category	Diameter	Mass
2005 FY₉ ("Easterbunny")	Cubewano	~1500 km	~4.0 × 10²¹ kg
Sedna	Scattered-Extended object	1180–1800 km	1.7-6.1 × 10²¹ kg
2003 EL₆₁ ("Santa")	Cubewano	~ 1500 km	~4.2 × 10²¹ kg
Quaoar	Cubewano	844 - 1260 km	1.0-2.6 × 10²¹ kg
2002 TC₃₀₂	Scattered disc object	420 - 1190 km	~7.8 × 10¹⁹ kg
Orcus	Plutino	~946 km	6.2 - 7.0 × 10²⁰ kg
Varuna	Cubewano	~874 km	~5.9 × 10²⁰ kg
2002 UX₂₅	Cubewano	~838 km	~7.9 × 10²⁰ kg
Ixion	Plutino	<822 km	~5.8 × 10²⁰ kg
2002 AW₁₉₇	Cubewano	~750 km	~5.2 × 10²⁰ kg
2002 TX₃₀₀	Cubewano	<709 km	1.6 - 3.7 × 10²⁰ kg

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)^[2] presented to the IAU stated that Charon could be considered a planet because:

Charon independently would satisfy the size and shape criteria for planetary status (and in the terms of the final resolution, for the status of dwarf planet)
Charon, on account of its large mass relative to Pluto, revolves with Pluto around a common barycentre located in space between Pluto and Charon rather than around a point located within Pluto.

This definition, however, was not preserved in the IAU's final resolution. It is unknown if it will be taken up at a future date. If a similar definition were to be adopted, Charon would be added to the list of dwarf planets.

The second, third, and fourth largest objects in the main asteroid belt (Vesta, Pallas and Hygiea) could be classified as dwarf planets if it is shown that their shape is determined by hydrostatic equilibrium. At present this has not been demonstrated conclusively.^[6] The Dawn probe, expected to enter orbit around Vesta in 2011, may provide evidence for or against dwarf planet in that case.

Size and mass of dwarf planets

The upper and lower limits to the size and mass of dwarf planets are not specified in the IAU resolution. There is strictly no upper limit, and an object larger or more massive than Mercury that is considered not to have cleared the neighborhood around its orbit may still 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 is undefined, and empirical observations suggest that it may vary according to the composition and history of the object. The original draft of IAU resolution 5 defined hydrostatic equilibrium shape as applying "to objects with mass above 510²⁰ kg and diameter greater than 800 km",^[2] but this language was not retained in the final resolution 5A that was passed.

According to some astronomers, the new definition could mean the addition of up to 45 new dwarf planets.^[8]^[9]

Orbital dominance

Main article: Clearing 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", that is, to remove smaller bodies whose orbits bring them nearby by collision, capture, or gravitational disturbance. This concept is combined with a concept of orbital dominance measured in terms of the ratio of the mass of a planetary candidate to the combined mass of all other objects in its vicinity. Dwarf planets are too small in mass to significantly alter their environment in the manner of a planet.

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. introduce a parameter Λ, 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. Stern and Levison found a gap of five orders of magnitude in Λ between the smallest terrestrial planets and the largest asteroids and KBOs:


Planetary discriminants
Body	Mass (M_E^*)	Λ/Λ_E^**	µ^***
Mercury	0.055	0.0126	9.110⁴
Venus	0.815	1.08	1.3510⁶
Earth	1.00	1.00	1.710⁶
Mars	0.107	0.0061	1.810⁵
Ceres	0.00015	8.710⁻⁹	0.33
Jupiter	317.7	8510	6.2510⁵
Saturn	95.2	308	1.910⁵
Uranus	14.5	2.51	2.910⁴
Neptune	17.1	1.79	2.410⁴
Pluto	0.0022	1.9510⁻⁸	0.077
Eris	0.0028	3.510⁻⁸	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.

Contention

A number of scientists expressed their disagreement^[10] with the currently adopted IAU definition of dwarf planet by means of car bumper stickers.

While accepting the characterisation of dwarf planet for Pluto and Eris (dwarf planet in this case meaning just a small planet), 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.^[11] Thus, he and his team will still refer to Pluto as the ninth planet. One should also note, that it will be in pages hosted by NASA and controlled by Stern's team, that the upcoming information and the first photographs of Pluto will be unveiled to the world. However, NASA has announced that it will use the new guidelines established by the IAU.^[12]

Prior to the 2006 IAU reclassification, several terms were suggested for bodies which are now formally cited as a dwarf planet, including minor planet, subplanet and planetoid.

Types of dwarf planets

The IAU's Resolution 6a^[3] 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^[14] and was abandoned in the final draft resolution (6b)^[15]; 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.^[16]

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 "Possible dwarf planets" (above) also 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.

References

1. ^ IAU 2006 General Assembly: Result of the IAU Resolution votes
2. ^ Draft Resolution 5 for GA-XXVI: Definition of a Planet.
3. ^ IAU 2006 General Assembly: Result of the IAU Resolution votes.
4. ^ Brown, M.E. et al. 2006. Satellites of the Largest Kuiper Belt Objects. Astrophysical Journal, 639:L43-L46 More accurate work based on Dysnomia's orbit in preparation.
5. ^ Draft Resolution 5 for GA-XXVI: Definition of a Planet.
6. ^ Three new planets may join solar system. New Scientist. Retrieved on 2006-08-16.
7. ^ Draft Resolution 5 for GA-XXVI: Definition of a Planet.
8. ^ Nine Planets Become 12 with Controversial New Definition. Space.com. Retrieved on 2006-08-16.
9. ^ What makes a planet?. Michael E. Brown. Retrieved on 2006-08-16.
10. ^ [1]
11. ^ Unabashedly Onward to the Ninth Planet.
12. ^ Hotly-Debated Solar System Object Gets a Name, NASA press release.
13. ^ IAU 2006 General Assembly: Result of the IAU Resolution votes.
14. ^ Astronomers divided over "planet" definition.
15. ^ The Final IAU Resolution on the definition of "planet" ready for voting.
16. ^ Draft definition, IAU press release.

External links

Dwarf Planets Page at NASA's Solar System Exploration
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

• • [ e] Footer SolarSystem

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Planets Dwarf planets Moons: Terrestrial Martian Jovian Saturnian Uranian Neptunian Plutonian Eridian
Small bodies: Meteoroids Asteroids/Asteroid moons (Asteroid belt) Centaurs TNOs (Kuiper belt/Scattered disc) Comets (Oort cloud)
See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the

Astronomical objects are significant physical entities, associations or structures which current science has confirmed to exist in space. This does not necessarily mean that more current science will not disprove their existence.
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Solar System or solar system^[a] consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 166 known moons,^[1]
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ORBit is a CORBA compliant Object Request Broker (ORB). The current version is called ORBit2 and is compliant with CORBA version 2.4. It is developed under the GPL license and is used as middleware for the GNOME project.
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The Sun

Observation data
Mean distance
from Earth 1.49610¹¹ m
(8.31 min at light speed)
Visual brightness (V) −26.74^m ^[1]
Absolute magnitude 4.
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Mass is a fundamental concept in physics, roughly corresponding to the intuitive idea of "how much matter there is in an object". Mass is a central concept of classical mechanics and related subjects, and there are several definitions of mass within the framework of relativistic
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Gravitation is a natural phenomenon by which all objects with mass attract each other. In everyday life, gravitation is most familiar as the agency that endows objects with weight.
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rigid body is an idealization of a solid body of finite size in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of external forces exerted on it.
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Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. The balance of these two forces is known as the hydrostatic balance.
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A sphere is a symmetrical geometrical object. In non-mathematical usage, the term is used to refer either to a round ball or to its two-dimensional surface. In mathematics, a sphere is the set of all points in three-dimensional space (R³
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In the end stages of planet formation, a planet will have cleared the neighbourhood of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational
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A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. Such objects are often called moons. Technically, the term could also refer to a planet orbiting a star, or even to a star orbiting a galactic center, but these
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2006 definition of "planet" by the International Astronomical Union (IAU) states that, in the solar system, a planet is a celestial body that:

is in orbit around the Sun,
has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and

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planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of
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Small Solar System Body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets nor dwarf planets:
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planetary system consists of the various non-stellar objects orbiting a star such as planets, moons, asteroids, meteoroids, comets, and cosmic dust.^[1]^[2] The Sun together with its planetary system, which includes Earth, is known as the Solar System.
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Ceres

Ceres
Discovery
Discovered by: Giuseppe Piazzi
Discovery date: January 1, 1801
Orbital characteristics
Epoch November 26, 2005
(JD 2453700.5)^[1]
Aphelion distance: 447,838,164 km
2.
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Pluto

Map of Pluto based on Charon eclipses, approximately true colour and giving the highest resolution currently possible
Discovery
Discovered by: Clyde W.
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Eris

Eris (centre) and Dysnomia (left of centre).
Hubble Space Telescope.
Discovery
Discovered by: M. E. Brown,
C. A. Trujillo,
D. L.
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The Minor Planet Center operates at the Smithsonian Astrophysical Observatory (SAO), which is part of the Center for Astrophysics (CfA) along with the Harvard College Observatory (HCO).
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asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter where 98.5% of the known minor planets' orbits can be found.
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Kuiper belt (pronounced IPA: /ˈkaɪpɚ/, to rhyme with "viper"),^[1] sometimes called the Edgeworth-Kuiper belt
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scattered disc (or scattered disk) is a distant region of our Solar System, thinly populated by icy minor planets known as scattered disc objects (SDOs), a subset of the broader family of trans-Neptunian objects (TNOs).
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kilogram or kilogramme (symbol: kg) is the SI base unit of mass. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of one liter of water.
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1 kilometre =
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US customary / Imperial units
0 ft 0 mi

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In physics, density is mass m per unit volume V—how heavy something is compared to its size. A small, heavy object, such as a rock or a lump of lead, is denser than a lighter object of the same size or a larger object of the same weight, such as pieces of
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tonne (t) or metric ton (M/T), also referred to as a metric tonne, is a measurement of mass equal to 1,000 kilograms. It is not an SI unit but is accepted for use with the SI.
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