The IAU decision in 2006 to introduce the concept of Dwarf Planets has resulted in four bodies (so far) being placed in this category. A Dwarf Planet is defined as a body which orbits the Sun, is sufficiently massive for its gravity to pull it into a near spherical shape, but is not sufficiently large to have cleared its orbit of other objects in orbit round the Sun. In the asteroid belt, only Ceres is considered large enough to be a dwarf planet.
By the 2006 definitions, planets are those larger bodies orbiting the Sun which have cleared the neighbourhood of their orbits of other bodies. Objects not massive enough to acquire a rounded shape are called smaller solar system bodies. These include the asteroids other than Ceres and comets.
Dwarf Planets lie between these other two groups. Although only four are recognised at present, it seems likely they will be added to in future years, particularly by the continuing discovery of Kuiper belt objects. The Kuiper belt is the region beyond the orbit of Neptune, 30 astronomical units from the Sun, out to some 55 astronomical units. Like the asteroid belt, it is thought to contain thousands of small objects, although probably made mostly of ices rather than rock.
The decision to introduce of dwarf planets reduced the number of (major) planets to 8, with Pluto being redesignated as a dwarf planet. The others are Ceres, the first asteroid to be discovered, Eris and Makemake.
The table shows some of the orbital data and some physical data for the four dwarf planets and, as a comparison, for the Moon.
|Date discovered||1 January 1801||18 February 1930||21 October 2003||31 March 2005|
|Discovered by:||Guiseppe Piazzi||Claude Tombaugh||Michael Brown at al||Michael Brown at al|
|Distance from Sun||2.55 to 2.997 AU||29.7 to 49.3 AU||37.8 to 97.6 AU||38.5 to 53.1 AU||1 AU|
|Period years||4.599||248.09||557||309.88||27.32 days|
|Diameter||975 km||2390 km||2600 ± 200 km||1500 ± 200 km||3475 km|
|Temperature||ca 167K, -100°C||ca 44K, -229°C||30 to 55K,-243 to -218°C||ca 30K, -243°C|
|Apparent magnitude||6.7 to 9.3||13.65 to 15.1||18.7||16.7 at opposition.|
|Charon, d = 1205 km
Nix d = ca 46 km
Hydra d = ca 61 km
d = 350±75 km