Planetary Systems includes topics such as Extrasolar and Solar planets and Planetology. A planetary system consists of the various non-stellar objects orbiting a star such as planets, moons, asteroids, meteoroids, comets, and cosmic dust. The Sun and its planetary system, which includes Earth, is known as the Solar System. Planetary systems are generally believed to form as part of the same process which results in star formation. Accepted theories today argue that a protoplanetary disk forms by gravitational collapse of a molecular cloud and then evolves into a planetary system by collisions and gravitational capture.