Solar System icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury), and, in
History of Solar System formation and evolution hypotheses of dynamics makes capture most unlikely as it requires special conditions. Swiss astronomer Louis Jacot (in 1951, 1962, 1981), like Weisacker and Ter Haar, continued the Cartesian idea of vortices but proposed a hierarchy of vortices or vortices within vortices, i.e., a lunar system vortex, a Solar System vortex, and a galactic vortex. He put forward the notion that planetary orbits are spirals, not circles or ellipses. Jacot also proposed the expansion of galaxies (stars move away from the hub), and that moons move away from their planets. He also maintained that planets were expelled, one at a time, from