Yellow card.

Thursday 24 August 2006 by perbosc

Pluto excluded from the solar system

The 2006 definition of "planet" by the International Astronomical Union (IAU) states that, in the solar system, a planet is a celestial body that:

is in orbit around the Sun,

has sufficient mass so that it assumes a hydrostatic equilibrium (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", whilst a non-satellite body fulfilling only the first criterion is termed a "small solar system body" (SSSB). The definition was a controversial one, and has drawn both support and criticism from different astronomers.

According to the definition, there are currently eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune ) and three dwarf planets (Pluto, Ceres Éris) known in the solar system.