4.2.4 Planets of the outer Solar System

The planets of the outer Solar System are Jupiter, Saturn, Uranus and Neptune. The former two are the giant planets, the latter two are also known as sub-giants. The planets are significantly more massive (compare Table 1 of Section 4.2.2.2) than the terrestrial planets and have considerably larger radii. Their mean densities are much smaller, between roughly 690 kg/m (Saturn) and 1640 kg/m (Neptune). The density values indicate volatile-rich compositions dominated by hydrogen and helium (Jupiter and Saturn) and water, ammonia, and methane (Uranus and Neptune). The latter compounds are often referred to as the ice component and Uranus and Neptune are sometimes referred to as the ice giants; H and He are also termed the gas component. Together with rock/iron, the major component of the terrestrial planets, the ice and the gas components are the main ingredients of simple planetary models. A recent review of the physics of the giant and sub-giant planets can be found in [07Gui]. The planets have numerous satellites (163 known at the time of this writing, compare Table 7 in 4.2.2.4) ranging in size from hundreds of meters to the radius of Ganymede (2632 km). The planets also have ring systems, the most prominent one is that of Saturn which is easily visible from Earth with small telescopes. Some of the smaller satellites are embedded in the ring systems and may feed the rings with particles such as e.g., Enceladus feeding Saturn’s E ring. Other satellites shepherd rings to sharpen their edges [06Esp] such as Prometheus and Pandora shepherd Saturn’s F ring. Data on the satellites and rings are collected in Sections 4.2.2.4 and 4.2.2.5.