A dwarf planet is a planetary-mass object that does not dominate its region of space (as a planet does) and is not a satellite. The prototype dwarf planet is Pluto.

The number of dwarf planets in the Solar System is unknown. This is because determining whether a body is in hydrostatic equilibrium requires close observation by spacecraft. The half-dozen largest candidates have either been visited by spacecraft (Pluto and Ceres) or have at least one known moon (Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into models of hydrostatic equilibrium.

(1) A planet is a celestial body that (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, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (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.

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies."

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