Planet.
An astronomical body in orbit around the Sun, or another star, which has a mass too small for it to become a star itself (less than about one-twentieth the mass of the Sun) and shines only by reflected light. Planets may be basically rocky objects, such as the inner planets - Mercury, Venus, Earth and Mars - or primarily gaseous, with a small solid core like the outer planets - Jupiter, Saturn, Uranus and Neptune. Together with Pluto, these are the major planets of the solar system. There are also within the solar system large numbers of minor planets, or asteroids. The presence of planets around stars other than the Sun has not been conclusively demonstrated but irregularities in the motion of certain stars (such as Barnard's Star), and statistical arguments, suggest that they probably do exist.