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Line 1: − + − + − *[[Eris]]+ − *[[Pluto]] − *[[Ceres]] − + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
Expansion of article
A '''dwarf planet''' is a body in space that is not large enough to be considered a [[planet]]. This includes meteorites and celestial bodies, even outside the range of the solar system. The oldest dwarf planets were created as the result of interstellar collision some ten billion years ago.
[[Image:Eris and Dysnomia.jpg|thumb|200px|right|[[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]]:{{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>}}
==See Also==
== Eris ==
: ''Main Article: [[Eris]]''
[[Category:Astronomy]]
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.
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:
{| class="wikitable"
|-
! Name
! [[Apsis|Perihelion]]
! [[Apsis|Aphelion]]
! [[Orbital eccentricity|Eccentricity]]
! [[Orbital period|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
|}
== References ==
<references/>
{{Astronomy portal}}
== See Also ==
* [[Planet]]
* [[Eris]]
* [[Pluto]]
* [[Ceres]]
[[Category:Dwarf planets]]