A planet is a celestial body that revolves around a star. Planets have enough mass to approximate a round shape, but too little mass to lead to nuclear fusion in their interiors, so they don't give off their own light.
A planet has to do three things: it must orbit a star, it must be large enough to have enough gravity to force it into a spherical shape, and it must be large enough to remove all objects of similar size near its orbit .