Artwork representing the formation of a planetary system around a star based upon the nebular hypothesis. According to this model a solar system derives from the contraction of a rotating disc- shaped nebula mainly formed by hydrogen, helium and dust grains. The contraction may be triggered by shock waves from a nearby supernova explosion or simply by the mutual gravitational attraction of the contents of the nebula. The central part of the nebula collapses into a protostar. Collisions between dust grains in the outer regions of the nebula lead to the formation of small bodies (few kilometres across), which then by accretion will develop into planets.