Artist impression of an exoplanet
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.