In 2006, the International Astronomical Union (IAU) introduced the new designation of a “dwarf planet”, defined as follows:
“A ‘dwarf planet’ is a celestial body that (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 (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.”
Prior to the introduction of this designation, the Kuiper-belt object Pluto had been considered to be a planet. However, since Pluto has not “cleared the neighbourhood around its orbit” this new definition demoted Pluto to the status of a dwarf planet.
The IAU’s new definition was brought in largely in response to the recent discovery of several other Pluto-sized objects beyond the orbit of Neptune. It is probable that there are hundreds of these dwarf planets in the Kuiper belt, and the furthest reaches of our solar system may be home to thousands more.
Critisism
The IAU’s definition of a dwarf planet was particularly contentious when it was first introduced, and there are many who still believe that Pluto should have its designation as a planet reinstated.
It has been pointed out that Earth would not be large enough to have swept its orbit clear – and, therefore, would not be defined as a planet – if it were the same distance from the Sun as Pluto, or even the same distance as Uranus. Indeed it is possible that a planet of any size could have its orbit disturbed through a gravitational encounter with another sufficiently large body, such that it might take up a new orbit that has not yet been swept clear of debris. In this scenario, the planet would presumably have to be reclassified as a dwarf planet, regardless of its size. This could be more of a problem in the future when classifying exoplanets in orbit around other stars, particularly double stars, where planetary orbits would be inherently unstable.
However, should Pluto be reclassified as a planet, then the other similarly-sized objects in the solar system would, presumably, have to be classified as planets too. The list of planets in our solar system could potentially grow to include hundreds, if not thousands, of objects.
Known Dwarf Planets
As of 2015, five objects have been positively identified as fulfilling the IAU’s criteria for designation as a dwarf planet.
In order of size, the designated dwarf planets are as follows:
Pluto
The Kuiper-Belt dwarf planet Pluto was discovered in 1930 and is named after the ancient Greek god of the underworld. See the main page on Pluto for more information.
Pluto has a mean radius of 1186 km – around 0.18 times the radius of the Earth and is still the ninth largest known object orbiting the Sun.
Eris
Although slightly smaller than Pluto by volume, Eris is over 25% more massive due to its much denser rocky composition. Eris is named after the ancient Greek goddess of strife and discord and was discovered in 2003.
Eris has a mean radius of 1163 km.
Makemake
Makemake (pronounced Maky Maky, Marky Marky or Markay Markay) is an icy Kuiper belt object. It is named after a god from the mythology of Easter Island because it was discovered shortly after Easter in 2005.
Makemake has a mean radius of around 715 km.
Haumea
Haumea was first imaged in 2003 and is named after a Hawaiian goddess. Haumea is thought to be composed almost entirely of solid rock and is possibly in a weak resonant orbit with Neptune.
Haumea has a mean radius of around 650 km.
Ceres
Ceres is the largest known object in the asteroid belt between the orbits of Mars and Jupiter. From the time of its discovery, on 1 January 1801, Ceres was considered to be a planet, until it was reclassified as an asteroid in 1850.
Ceres has a mean radius of 473 km.
Potential Dwarf Planets
Other potential dwarf planets, which, as of 2015, have not yet been officially recognised as such by the IAU, include:
- Orcus
- 2002 MS4
- Salacia
- Quaoar
- 2007 OR10
- Sedna