A planet is a large body that orbits a star. For an object to be a planet, it must be massive enough that gravity has pressed it into a spherical or circular shape. It must also be large enough that gravity engulfs any rocky or icy object in its path or orbit around the star. Scientists believe that planets begin to form when a dense cloud of dust and gas, called a nebula, orbits around a newly formed star. Gradually, gravity causes the pieces of matter in the nebula to clump together. Slowly,