A planet is a type of celestial body, a large collection of matter in space, in orbit around a star but not having achieved thermonuclear fusion. There are many planets, though only the eight that are in our solar system were known until the discovery of exoplanets in the 1990s. The eight planets of the Solar System are, in order of distance from the sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto was discovered in 1930, and was traditionally regarded as a planet until other, more massive objects were found in its vicinity. This led to the IAU establishing an official definition for a planet, re-categorizing Pluto as a dwarf planet.


In 2006, the International Astronomical Union voted on an official definition of the term "planet". A planet has been henceforth defined as a body that:

  • Is in orbit around a sun;
  • Has achieved a hydrostatic equilibrium;
  • Has "cleared the neighborhood" (its path) of other foreign bodies or particles.

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