- (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.
List of dwarf planets
The IAU has officially identified five (or 7) celestial bodies that have immediately received "dwarf planet" classification:
|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|
| 1.305×1022 kg|
| ~1.6×1022 kg |
| Mean equatorial radius*|
| 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)
| Orbital radius* (AU)|
| Orbital period*(a)|
(in sidereal years)
| Mean orbital speed|
| Inclination of the equator from the orbit|
(see Axial tilt)
|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:
|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|
(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|
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:
- 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 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.
- 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:
|Body|| Mass (ME*)|| Λ/ΛE**||µ***|
|90377 Sedna||8.3 x10||Unknown||Unknown|
*ME in Earth masses.
**Λ/ΛE = 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.
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
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.
- 2006 redefinition of planet
- Classical planet
- Small solar system body
- Trans-Neptunian object
- List of solar system objects by planetary discriminant
- Clearing the neighbourhood
- ↑ IAU 2006 General Assembly: Result of the IAU Resolution votes
- ↑ 2.0 2.1 2.2 Draft Resolution 5 for GA-XXVI: Definition of a Planet.
- ↑ 3.0 3.1 IAU 2006 General Assembly: Result of the IAU Resolution votes.
- ↑ 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.
- ↑ Three new planets may join solar system. New Scientist. Retrieved on 2006-08-16.
- ↑ Nine Planets Become 12 with Controversial New Definition. Space.com. Retrieved on 2006-08-16.
- ↑ What makes a planet?. Michael E. Brown. Retrieved on 2006-08-16.
- ↑ http://news.bbc.co.uk/2/hi/science/nature/5283956.stm
- ↑ Unabashedly Onward to the Ninth Planet.
- ↑ Hotly-Debated Solar System Object Gets a Name, NASA press release.
- ↑ Astronomers divided over "planet" definition.
- ↑ The Final IAU Resolution on the definition of "planet" ready for voting.
- ↑ Draft definition, IAU press release.
- 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
|The Sun · Mercury · Venus · Earth · Mars · Ceres* · Jupiter · Saturn · Uranus · Neptune · Pluto* · Eris* · Sedna*|
|Planets · Dwarf planets · Moons: Terran · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian|
|Pluto' * Ceres * Eris * Haumea * Makemake|
|Small bodies: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)|
|planets with '*' are dwarf's but listed between 'real planets'.|
|See also astronomical objects and the solar system's list of objects, sorted by radius or mass.|