A model of star and planet formation in which a nebula contracts under the force of gravity, eventually flattening into a spinning disk with a central bulge. A protostar forms at the nebula's center. As matter condenses around the protostar in the bulge, planets are formed from the spinning matter in the disk. This theory is widely accepted to account for the formation of stars and planetary systems such as ours. The first version of the nebular hypothesis was proposed in 1755 by the German philosopher Immanuel Kant and modified in 1796 by Pierre Laplace.♦ The nebula that according to this hypothesis condensed to form the solar system is called the solar nebula.
