A planet (from Ancient Greek αστήρ πλανήτης (astēr planētēs), meaning "wandering star") is an astronomical object orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of deities In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition has been remains disputed by some scientists because it excludes many objects of planetary mass based on where or what they orbit. While eight of the planetary bodies discovered before 1950 remain "planets" under modern definition, some celestial bodies, such as Ceres, Pallas, Juno, Vesta (each an object in the Solar asteroid belt) and Pluto (the first-discovered trans-Neptunian object), that were once considered planets by the scientific community are no longer viewed as such.
Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Additionally, the IAU accepts five dwarf planets, with many others under consideration, and hundreds of thousands of small Solar System bodies.
Courtesy of Wikipedia:http://www.wikipedia.org/