Nursery for Planets
This chapter illustrates how the solar system has a decidedly two-dimensional aspect to it. The orbits of the eight major planets all lie in almost the same plane, deviating by no more than seven degrees. Bodies in the asteroid belt and the Kuiper belt stray a little further afield, but these belts are arranged like flattened donuts, aligned with the same plane as the planets. Immanuel Kant and Pierre-Simon de Laplace noted the planar nature of the solar system and used this as the basis for their nebular theories in which the solar system grew out of a flattened disk of matter. Young stars like those in the constellation Orion are often surrounded by disk-shaped clouds of gas and dust. Astronomers quickly dubbed these “protoplanetary” disks, assuming that they will someday form planetary systems.