Planet

This article is about the astronomical term. See also the related but distinct term dwarf planet.

For the astrological concept, see Planets in astrology. For other uses, see Planet (disambiguation).

The eight planets and three dwarf planets of the Solar System. (Sizes to scale.)

The International Astronomical Union defines "planet" as a celestial body that, within the Solar System,^[1]

(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;

or within another system,^[2]

(i) is in orbit around a star or stellar remnants;

(ii) has a mass below the limiting mass for thermonuclear fusion of deuterium; and

(iii) is above the minimum mass/size requirement for planetary status in the Solar System.

Our solar system is thus considered to have eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Under a separate resolution, it is also considered to have three dwarf planets: Ceres, Pluto, and Eris. There have been more than two hundred planets discovered orbiting other stars to date.^[3]

Historically, there has been no formal scientific definition of "planet" and without one, the Solar System had been considered to have various planets over the years. This changed when a resolution covering planets within our solar system was formally adopted by the IAU in 2006, limiting the number to eight. However, the IAU's position on those in other systems remains only a working definition in place since 2003, and as such, is easily subject to change. The IAU has not yet taken a position on free-floating objects of planetary mass outside star systems, other than to exclude those in young star clusters.