Message #20716

[mccoy]

The definition of planet set in Prague, Czech Republic in August 2006 by the International Astronomical Union (IAU) states that, in the Solar System, a planet is a celestial body which:

is in orbit around the Sun,
has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
has "cleared the neighborhood" around its orbit.
A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet". According to the IAU, "planets and dwarf planets are two distinct classes of objects". A non-satellite body fulfilling only the first criterion is termed a "small Solar System body" (SSSB). Initial drafts planned to include dwarf planets as a subcategory of planets, but because this could potentially have led to the addition of several dozens of planets into the Solar System, this draft was eventually dropped. The definition was a controversial one and has drawn both support and criticism from different astronomers, but has remained in use.