Dwarf planet

Nine likeliest^[a] dwarf planets
and dates of discovery

Ceres (1801)

Pluto (1930)

Quaoar (2002)

Sedna (2003)

Orcus (2004)

Haumea (2004)

Eris (2005)

Makemake (2005)

Gonggong (2007)

A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like the eight classical planets of the Solar System. The prototypical dwarf planet is Pluto, which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.

Dwarf planets are capable of being geologically active, an expectation that was borne out in 2015 by the Dawn mission to Ceres and the New Horizons mission to Pluto. Planetary geologists are therefore particularly interested in them.

Astronomers are in general agreement that at least the nine largest candidates are dwarf planets – in rough order of size, Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Ceres, Orcus, and Sedna. Considerable uncertainty remains over the tenth largest candidate Salacia, which may thus be considered a borderline case. Of these ten, two have been visited by spacecraft (Pluto and Ceres) and seven others have at least one known moon (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into geophysical models in an attempt to determine the nature of these worlds. Only one, Sedna, has neither been visited nor has any known moons, making an accurate estimate of mass difficult. Some astronomers include many smaller bodies as well,^[1] but there is no consensus that these are likely to be dwarf planets.

The term dwarf planet was coined by planetary scientist Alan Stern as part of a three-way categorization of planetary-mass objects in the Solar System: classical planets, dwarf planets, and satellite planets. Dwarf planets were thus conceived of as a category of planet. In 2006, however, the concept was adopted by the International Astronomical Union (IAU) as a category of sub-planetary objects, part of a three-way recategorization of bodies orbiting the Sun: planets, dwarf planets, and small Solar System bodies.^[2] Thus Stern and other planetary geologists consider dwarf planets and large satellites to be planets,^[3] but since 2006, the IAU and perhaps the majority of astronomers have excluded them from the roster of planets.

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