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From Citizendium, the Citizens' Compendium
Dwarf planets are celestial objects orbiting a star that are massive enough to obtain a round shape but too small to clear their orbital path of other celestial bodies. Our Solar System has five celestial bodies currently recognized as dwarf planets by the International Astronomical Union.
Definition of dwarf planet
For a celestial body to qualify as a planet it has to meet three criteria established by the International Astronomical Union (IAU). In a resolution passed by the IAU on August 24, 2006, a planet is a celestial body that
- (a) is in orbit around a 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 cleared the neighborhood around its orbit.
A dwarf planet can only meet criteria (a) and (b). In other words it is not massive enough to clear other bodies from its immediate neighborhood in its orbital path. It must also meet a fourth: it is not a satellite of another planet. There is another classification of celestial bodies smaller than a dwarf planet, small Solar-System bodies (SSSB). They are simply in orbit around the sun and are not massive enough to form a round shape. They include all the comets and all the minor planets.
Acknowledged dwarf planets
So far, only two bodies have been confirmed to satisfy the criteria for dwarf planets by direct observation: Ceres and Pluto. For other bodies for the time being classification has to be based on likelihoods based on indirect evidence. The International Astronomical Union currently provisionally classifies three others as dwarf planets: Haumea, Makemake and Eris. Some astronomers are less cautious and recognize various other dwarf planets. The IAU maintains a dwarf planet watch list for celestial bodies under study and awaiting classification. In order of size, the five currently listed by the IAU are:
Eris was discovered orbiting in the scattered disc in July 2005, by a team of astronomers at CalTech. It has a diameter of 3,000 km (1,850 miles) making it 700 km larger than Pluto. Eris's size was calculated by measuring the heat it reflects. It is now known to be the largest of the dwarf planets and is the largest object discovered in our solar system since the discovery of Neptune and its moon Triton in 1846.
At this time, Eris is the furthest known object in orbit around the sun (other objects with more eccentric orbits will travel further out, of course). It is nearly 1.609 X1011 km (10 billion miles) from the sun. This is nearly three times the distance of Pluto and it takes nearly twice as long to orbit the sun as Pluto. It has one known moon, Dysnomia.
Eris was provisionally named UB313, or Xena. The official name, Eris, was mooted by the team that originally sighted the dwarf planet, Mike Brown of Caltech, Chad Trujillo of Gemini Observatory, and David Rabinowitz of Yale University. The term was officially adopted by the IAU and announced 13 September 2006.
Pluto, regarded as the ninth planet when discovered in 1930, is an object orbiting the sun in the Kuiper Belt, a disc-like zone beyond the orbit of Neptune, composed of a vast number of asteroids and potential dwarf planets. Some of the objects in the Kuiper Belt have diameters of at least 1,000 km. The Kuiper Belt is also a possible source of comets. Pluto has three known moons, Hydra and Nix, and Charon.
Ceres, which lies in the asteroid belt between Mars and Jupiter, was discovered on 1 January 1801 by Giuseppe Piazzi. Originally regarded as a planet, it was classified as an asteroid until August 24, 2006, by the IAU (International Astronomic Union). It is named after the Roman goddess.
Ceres has a diameter of about 960 km and has a mass that totals approximately one third of the asteroid belt’s total.