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
Lucky Starr and the Moons of Jupiter of Eternity". Consequently, there are small inconsistencies between the various novels. In the first, Ganymede is mentioned as a settled world of the Solar System, although in later novels Ganymede has no permanent settlement, and has even served as the location of a secret Sirian base. The first man on Mars is named Wingrad in "David Starr, Space Ranger", but is named Ching in "Moons of Jupiter". The name of the state that rules the Solar System also changes from novel to novel. In "Pirates of the Asteroids", it is the Terrestrial Empire; in "Oceans of Venus", it is the