Dwarf (astronomy)

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The International Astronomical Union (IAU), the official scientific body for astronomical nomenclature, defines a "dwarf planet" as a celestial body within the Solar System that satisfies these four conditions:[1]

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.[2]

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

  • Ceres - 975�909 km in diameter, discovered January 1, 1801
  • Pluto - 2306�20 km in diameter, discovered February 18, 1930
  • Eris - 2400�100 km in diameter, discoverd January 5, 2005


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
Template:Mpl- ("Easterbunny") Cubewano 1600 � 2000? km unknown
Template:Dp Plutino 840 - 1880 km6.2 - 7.0 � 1020 kg
Template:Dp Scattered-Extended object 1180�1800 km1.7-6.1 � 1021 kg
Template:Mpl- ("Santa") Cubewano ~ 1500 km ~4.2 � 1021 kg
Template:Dp 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
Template:Mpl Scattered disc object ≤ 1200 kmunknown
Template:Dp Cubewano ~936 km~5.9 � 1020 kg
Template:Mpl Cubewano ~910 km ~7.9 � 1020 kg
Template:Mpl Cubewano <900 km unknown
Template:Dp 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)[2] 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.[4]

Contents

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 5×1020 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.[5][6]

See also


External links


References

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