The Outer Planets

Among the first facts that most people learn about the solar system is that its planets fall into two distinct categories. The innermost four—Mercury, Venus, Earth and Mars—are the terrestrial planets, with solid rocky surfaces. The outermost four—Jupiter, Saturn, Uranus and Neptune—are the gas giants—objects which are physically much larger and the majority of whose mass is made up of gas rather than solid material (Fig. 6.1).