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A '''dwarf planet''' is a satellite of the [[Sun]] that meets the following criteria now set by the [[International Astronomical Union]]:{{Cquote|(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.<ref>"[http://www.iau.org/iau0602.423.0.html IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting]," ''International Astronomical Union'', 2005. Accessed January 14, 2008.</ref>}}

A '''dwarf planet''' is a satellite of the [[Sun]] that meets the following criteria now set by the [[International Astronomical Union]]:{{Cquote|(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.<ref>"[http://www.iau.org/iau0602.423.0.html IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting]," ''International Astronomical Union'', 2005. Accessed January 14, 2008.</ref>}}

The concept of "dwarf planet" properly dates to the discovery of the scatter-disk object named [[Eris]] on January 5, 2005. With the confirmation of the identification of Eris, a formal debate began on the subject of what does, and what does not, constitute a [[planet]]. The community of [[Astronomy|astronomers]] decided that relying on history and tradition simply would not serve. The eight celestial bodies that now remain under the present definition of ''[[planet]]'' do not share their orbits with any other objects. The objects now called dwarf planets do not have this distinction. And yet these objects are significantly heavier than mere [[asteroid]]s, in that they are so heavy that their own weight, and more particularly their own gravity, forces them to assume the round or nearly-round shape that all planets have.

The concept of "dwarf planet" properly dates to the discovery of the scatter-disk object named [[Eris]] on January 5, 2005. With the confirmation of the identification of Eris, a formal debate began on the subject of what does, and what does not, constitute a [[planet]]. The community of [[Astronomy|astronomers]] decided that relying on history and tradition simply would not serve. The eight celestial bodies that now remain under the present definition of ''[[planet]]'' do not share their orbits with any other objects. The objects now called dwarf planets do not have this distinction. And yet these objects are significantly heavier than mere [[asteroid]]s, in that they are so heavy that their own weight, and more particularly their own gravity, forces them to assume the round or nearly-round shape that all planets have.

[[Image:Pluto sri.jpg|thumb|150 px|left|True-color image of [[Pluto]]]]Eris has another distinction that forced the debate: it is more massive even than [[Pluto]] (by twenty-seven percent) and is therefore the heaviest dwarf planet yet found. The mass of Eris is inferred from the orbital parameters, including the [[Apsis|apsides]] and period, of the small body that orbits Eris. If Pluto could still be called a planet, then Eris deserved that distinction as well.

Eris has another distinction that forced the debate: it is more massive even than [[Pluto]] (by twenty-seven percent) and is therefore the heaviest dwarf planet yet found. The mass of Eris is inferred from the orbital parameters, including the [[Apsis|apsides]] and period, of the small body that orbits Eris. If Pluto could still be called a planet, then Eris deserved that distinction as well.

In 2006, the International Astronomical Union settled the issue. They determined that Eris was not a planet, and neither was Pluto. But the criteria they set also provoked yet another reassessment of the status of [[Ceres]], the largest and first-discovered object in the [[asteroid]] belt. Upon consideration, the IAU declared that Ceres was a dwarf planet as well.

In 2006, the International Astronomical Union settled the issue. They determined that Eris was not a planet, and neither was Pluto. But the criteria they set also provoked yet another reassessment of the status of [[Ceres]], the largest and first-discovered object in the [[asteroid]] belt. Upon consideration, the IAU declared that Ceres was a dwarf planet as well.