The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body that:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has "cleared the neighbourhood" around its orbit.

A non-satellite body fulfilling only the first two of these criteria (such as Pluto, which had hitherto been considered a planet) is classified as a dwarf planet. According to the IAU, "planets and dwarf planets are two distinct classes of objects" – in other words, "dwarf planets" are not planets. A non-satellite body fulfilling only the first criterion is termed a small Solar System body (SSSB). An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The decision was a controversial one, and has drawn both support and criticism from astronomers.

The IAU has stated that there are eight known planets in the Solar System. It has been argued that the definition is problematic because it depends on the location of the body: if a Mars-sized body were discovered in the inner Oort cloud, it would not have enough mass to clear out a neighbourhood that size and meet criterion 3.

The definition distinguishes planets from smaller bodies and is not applicable outside the Solar System. To date, there is no accepted definition of extrasolar planets, or exoplanets. In 2007, an IAU working group issued a position statement that proposes to distinguish exoplanets from brown dwarfs on the basis of mass, but there has been no IAU-wide resolution or vote associated with this position statement. A separate proposal to extend the IAU definition to exoplanets has not been formally reviewed by the IAU.