Dwarf planet

The International Astronomical Union (IAU), the official scientific body for astronomical nomenclature, defines a "dwarf planet" as a celestial body that, within the Solar System,

(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 (near-spherical) shape;

(c) has not cleared the neighbourhood around its orbit; and

(d) 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". As defined, the term "dwarf planet" does not apply to other planetary systems.

Five dwarf planets are currently recognized: Ceres, Pluto, Eris, Haumea and Makemake. Other bodies might be added to the list once it has been determined whether criterion (b) is fulfilled.

List of dwarf planets

The IAU has officially identified five (or 7) celestial bodies that have immediately received "dwarf planet" classification:

Dwarf planets

Name	Ceres	Pluto	Eris	Haumea	Makemake
MPC number	1	134340	136199	136108	136472
Region of Solar System	Asteroid belt	Kuiper belt	Scattered disc
Diameter	975×909 km	2306±20 km	2400±100 km	1440±50 km	1420±30
Mass in kg compared to Earth	9.5×1020 kg .00016	1.305×1022 kg .0022	~1.6×1022 kg .0027	21} .0007	21} .0005
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	2.6-3.3	~2.0
Equatorial gravity (in m/s2)	0.27	0.60			~0.4
Escape velocity (in km/s)	0.51	1.2			~0.75
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	34.72-51.54z 43.132	38.51-53.07 45.791
Orbital period*(a) (in sidereal years)	4.599	248.09	557	283.28	309.88
Mean orbital speed (in km/s)	17.882	4.666	3.437	4.484	4.419
Orbital eccentricity	0.080	0.24880766	0.44177	0.19501	0.159
Orbital inclination	10.587°	17.14175°	44.187°	28.22°	28.96°
Inclination of the equator from the orbit (see Axial tilt)	4°	119.61°
Mean surface temperature (in K)	167	40	30	<50	30-35
Number of natural satellites	0	4	1	2
Date of discovery	January 1, 1801	February 18, 1930	January 5, 2005	December 28, 2004	March 31, 2005

*Measured relative to the Earth.

Additionally, there are several bodies potentially qualifying as "dwarf planets". Among these, the following are known or thought to be greater than around 750 km in diameter:

Possible dwarf planets

Name	Category	Diameter	Mass
2005 FY9 ("Easterbunny")	Cubewano	1600 – 2000? km	unknown
Orcus	Plutino	840 - 1880 km	6.2 - 7.0 × 1020 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	989 - 1346? km	1.0-2.6 × 1021 kg
Charon (satellite of Pluto)	Plutino	1207 km ± 3 km	(1.52±0.06)×1021 kg
2002 TC302	Scattered disc object	≤ 1200 km	unknown
Varuna	Cubewano	~936 km	~5.9 × 1020 kg
2002 UX25	Cubewano	~910 km	~7.9 × 1020 kg
2002 TX300	Cubewano	<900 km	unknown
Ixion	Plutino	<822 km	unknown

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) presented to the IAU stated that Charon could be considered a planet because:

1. 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)
2. 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 asteroids (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.

Size and mass of dwarf planets

The upper and lower limits to the size and mass of dwarf planets are not specified in IAU resolution 5A. 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 5×10²⁰ kg and diameter greater than 800 km", 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.

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 (ME*)	Λ/ΛE**	µ***
Mercury	0.055	0.0126	9.1×104
Venus	0.815	1.08	1.35×106
Earth	1.00	1.00	1.7×106
Mars	0.107	0.0061	1.8×105
Ceres	0.00015	8.7×10−9	0.33
Jupiter	317.7	8510	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	0.0022	1.95×10−8	0.077
Eris	0.005	3.5×10−8	0.10
90377 Sedna	8.3 x10	Unknown	Unknown

*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 with the currently adopted IAU definition of "dwarf planet" by means of car bumper stickers.

While accepting the characterization 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. 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.

Types of dwarf planets

Illustration of the size estimates of the largest trans-Neptunian objects including Pluto and Eris. Other objects may meet the definition of a dwarf planet.

The 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. Pluto's largest satellite, Charon would qualify if it were to be classed as a dwarf planet in its own right. 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.

See also

• 2006 redefinition of planet
• Classical planet
• Mesoplanet
• Small solar system body
• Trans-Neptunian object
• List of solar system objects by planetary discriminant
• Clearing the neighbourhood

References

• The IAU press release and supporting web site, 16 August 2006
• The Scientific American Blog on Plutons, 18 August 2006

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