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Artist impression of an exoplanet
Credit: NASA

A planet is an object which orbits a star.

Planets must have three things to be classed as planets and not other objects, such as dwarf planets, these are:

  • Be spherical in shape, rounded by its own gravity.
  • Be small enough not to becomes stars themselves, by carrying out nuclear fusion.
  • Have cleared it's local area of other objects, such as asteroids.

Planets tend to be divided into terrestrial planets, with a solid surface, and gas giants, which are made of layers of gas with small rocky cores.

Planets which orbit other stars (i.e. NOT the Sun) are called exoplanets.