Planet
Planet, any major celestial body that orbits a star and does not emit visible light of its own but instead shines by reflected light. Smaller bodies that also orbit a star and are not satellites of a planet are called asteroids or planetoids. In the solar system, there are nine planets: Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , Neptune , and Pluto . Planets that orbit stars other than the Sun are collectively called extrasolar planets . Some extrasolar planets are nearly large enough to become stars themselves. Such borderline planets are called brown dwarfs . See also Planetary Science .