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Dwarf planet

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Eris and its satellite, Dysnomia.
A dwarf planet is a satellite of the Sun that meets the following criteria now set by the International Astronomical Union:
(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.[1]



Main Article: Pluto

The first of the dwarf planets ever discovered was Pluto. At the time, no astronomer thought to classify Pluto any differently from any other planet. Yet Pluto was always an anomaly among planets. Not only was it the smallest planet, but it was also a rocky or "terrestrial" planet in a region that ought to have only gas giants within it, according to the nebula hypothesis. Some astronomers even speculated that the Sun had captured Pluto from outside its system.


Main Article: Eris

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 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 asteroids, 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.

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 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.

Today the list of dwarf planets is limited to these three: Eris, Pluto, and Ceres.


List of all known dwarf planets, from the innermost to the outermost:
Name Perihelion Aphelion Eccentricity Sidereal year Inclination Mass Sidereal day
Ceres 3809820115292.547 AU380,982,011.529 km
236,731,246.725 mi
4468273011322.987 AU446,827,301.132 km
277,645,612.829 mi
0.079541620.0795 1680.9731634.602 a1,680.973 da 0.18476760574610.586 °0.185 rad
11.763 grad
9.47E+200.0129 M☾9.47e+20 kg
1.584675e-4 M⊕
326709.075 h0.378 da
Pluto 4.436824613E+1229.658 AU4,436,824,613 km
2,756,914,999.528 mi
7.375927931E+1249.305 AU7,375,927,931 km
4,583,189,132.342 mi
0.248807660.249 90614.8725248.09 a90,614.873 da 0.29917997705417.142 °0.299 rad
19.046 grad
1.305E+220.178 M☾1.305e+22 kg
0.00218 M⊕
-551856.672-153.294 h-6.387 da
Eris 5.6742472091E+1237.93 AU5,674,247,209.1 km
3,525,813,753.368 mi
1.45902802611E+1397.53 AU14,590,280,261.1 km
9,065,979,840.917 mi
0.440.44 203590.35557.4 a203,590.35 da 0.77072867103444.159 °0.771 rad
49.066 grad
1.66E+220.226 M☾1.66e+22 kg
0.00278 M⊕
288008 h0.333 da
Use a JavaScript-enabled browser to view this element. Browse the result list directly.DECADECENTURYCeres1801-01-01T00:00:000Date of discovery 1 January 18011 January 1801
16 Teveth 5561 He
15 Teveth 5804 AM

Discoverer Giuseppe Piazzi
Name origin Roman goddess of agriculture
Celestial class Solar system, Dwarf planet
Pluto1930-01-01T00:00:000Date of discovery 19301930
5690 He
5933 AM

Discoverer Clyde W. Tombaugh
Name origin Greco-Roman god of wealth and the underworld
Celestial class Solar system, Dwarf planet
Eris2003-10-21T00:00:000Date of discovery 21 October 200321 October 2003
25 Tishrei 5764 He
25 Ethanim 6007 AM

Discoverer David Rabinowitz, Chad Trujillo, Michael Brown
Name origin Greek goddess of discord
Celestial class Solar system, Dwarf planet


  1. "IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting," International Astronomical Union, 2005. Accessed January 14, 2008.
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