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
Numerical model of the Solar System and only practical on an electronic computer. As such only the latter was used in former times. Strictly speaking not much less calculation intensive, but it was possible to start with some simple approximations and then to add perturbations, as much as needed to reach the wanted accuracy. In essence this mathematical simulation of the Solar System is a form of the "N-body problem". The symbol N represents the number of bodies, which can grow quite large if one includes 1 sun, 8 planets, dozens of moons and countless planetoids, comets and so forth. However the influence of the sun