Exoplanet, also known as an extrasolar planet, is a planet outside of the Solar System that orbits a star other than the Sun. Exoplanets vary in size (Draugur, the smallest known exoplanet, is only twice the mass of the Moon, while DENIS-P is about 29 times the mass of Jupiter). Exoplanets are primary candidates in the search for extraterrestrial life, especially ones located in a star's habitable zone.