A planet, as defined by the International Astronomical Union, is a celestial body meeting the following requirements:

For a body in the solar system:

  • is in orbit around the Sun
  • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
  • has cleared the neighbourhood around its orbit

For a body within another planetary system:

  • is in orbit around a star or the remnants of one
  • has a mass below the limiting mass for thermonuclear fusion of deuterium
  • is above the minimum mass/size requirement for planetary status in the Solar System.

Therefore, the solar system counts 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In addition to these eight, three dwarf planets are also known: Ceres, Pluto and Eris. As for extrasolar planets, 241 are known as of September 2007.

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