Dwarf planet

Dwarf planets are celestial objects orbiting a sun that are massive enough to obtain a round shape but too small to clear their orbital path of other celestial bodies. Our solar system currently has three celestial bodies classified as dwarf planets.

Definition of dwarf planet

For a celestial body to qualify as a planet is 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 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

There are currently three celestial bodies classified as dwarf planets in our solar system and there are a number of others that will possibly be classified in the future. The IAU maintains a dwarf planet watch list for celestial bodies under study and awaiting classification. In order of size, they are:

Eris

Eris was discovered orbiting in the scattered disc in July 2005, by Mike Brown and a team of astronomers at CalTech. It is larger than Pluto. Eris was provisionally named UB313, or Xena, but has officially been named Eris by the IAU. Eris 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. Eris 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.

Pluto

Pluto, 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 astreroids 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

Ceres was discovered in January, 1801 by Giuseppe Paizzi. Originally classified 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. Ceres which lies in the asteroid belt between Mars and Jupiter has a diameter of about 960 km and has a mass that totals approximately one third of the asteroid belt’s total mass.