Since the time of Copernicus we have understood that the planets of the solar system orbit the Sun in nearly circular and coplanar paths. Based on this observation, Kant (1755) proposed that the planets formed out of a thin disc that surrounded the Sun earlier in its history. In Kant’s theory, as in the currently favored paradigm, particles in the disc followed circular Keplerian orbits around the Sun and aggregated into successively larger bodies that eventually became planets.