Dwarf Planet

A dwarf planet is a planetary-mass object that is neither a planet nor a satellite. More explicitly, the International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravitational rather than mechanical forces (that is, it has sufficient mass to overcome its internal compressive strength and achieve hydrostatic equilibrium, and is thus an ellipsoid in shape), but that unlike a planet has not cleared its orbital region of other objects. The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by Mike Brown, who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern, who had coined the term dwarf planet in 1990.

The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake, and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1) are to be named under the assumption that they are dwarf planets. The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.

It is suspected that another hundred or so known objects in the Solar System are dwarf planets. Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered. Individual astronomers have accepted several additional bodies as dwarf planets, and Mike Brown published in August 2011 a list of 390 candidate objects, organized in categories from "nearly certainly" to "possibly" meeting the IAU's criteria, along with his classification methodology. Brown identifies nine known objects – the five accepted by the IAU plus 2007 OR₁₀, Sedna, Quaoar, and Orcus – as "virtually certain", with another two dozen highly likely, and there are probably a hundred or so such objects in total.

The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.