Dwarf Planet

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:

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(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.

”

Eris

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.

Table

List of all known dwarf planets, from the innermost to the outermost:

Name	Perihelion	Aphelion	Eccentricity	Sidereal year	Inclination	Mass	Sidereal day
Ceres	2.545 AU	2.987 AU	0.0798	4.599 a	10.587 °	0.0158% earth	0.378 da
Pluto	29.658 AU	49.305 AU	0.249	248.09 a	17.142 °	0.218% earth	-6.387 da
Eris	37.77 AU	97.56 AU	0.442	577 a	44.187 °	0.278% earth	0.333 da

See Also

• Planet
• Eris
• Pluto
• Ceres