A planet is a celestial body of a certain shape and size with certain characteristics. Some of these characteristics include gravity creating a sphere-like shape and being small enough to be unable to create thermonuclear fusion. Most planets, but not all, orbit one or more stars.
A planet usually has a diameter above 100 kilometers; celestial bodies below this size are usually classified as planetoids or asteroids. Their surfaces, compositions and masses can vary from planet to planet. For example, some planet surfaces may be completely or mostly comprised of water, while other planet surfaces are completely gaseous.