In the 1700's, Immanuel Kant and the Marquis de Laplace proposed that the solar system collapsed from a gaseous medium of roughly uniform density. A flattened gaseous disk, which became known as the protosolar nebula, formed out of this cloud. The Sun contracted out of material at the center of the disk; the planets condensed in the outer portions.

In the 1950's, Jan Oort hypothesized a vast cloud of comets surrounding the Sun at a distance of 50,000 AU. At about the same time, Edgeworth and Kuiper independently proposed that smaller disk-shaped clouds of comets might orbit the Sun just beyond the orbit of Neptune, which orbits the Sun at a distance of roughly 30 AU .

Both theories tried to explain different classes (families) of cometary orbits around the Sun, with long-period comets originating in the Oort cloud and short-period comets originating in the Edgeworth-Kuiper Belt. As computer caclulations of comet orbits became more sophisticated in the 1970's and 1980's, both of these theories became more broadly accepted.