IAU definition of planet

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:

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). An alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The definition was a controversial one, and has drawn both support and criticism from different astronomers, but has remained in use.

According to this definition, there are eight known planets in the Solar System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar System, where smaller bodies cannot be found yet.Extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger.

Before the discoveries of the early 21st century, astronomers had no real need for a formal definition for planets. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of smaller bodies such as asteroids and comets. Pluto was thought to be larger than Mercury.

In 1978, the discovery of Pluto's moon Charon radically changed this picture. By measuring Charon's orbital period, astronomers could accurately calculate Pluto's mass for the first time, which they found to be much smaller than expected. Pluto's mass was roughly one twenty-fifth of Mercury's, making it by far the smallest planet, smaller even than the Earth's Moon, although it was still over ten times as massive as the largest asteroid, Ceres.

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