Preamble

As early astronomers recognized, the planets are orbiting the Sun on paths that are nearly circular and coplanar, with motions in the same sense as the Sun's rotation, making it difficult to avoid the conclusion that the formation of the Sun led directly to its planetary system. A comparison of the planets (Table 1.1) shows that the Earth belongs to an inner group of four, which are much smaller and denser than the outer four giant planets. For this reason the inner four are referred to as the terrestrial (Earth-like) planets. The outer four large planets are gaseous, at least in their visible outer regions, but they have solid satellites with very varied external appearances and a wide range of mean densities, all much lower than those of the terrestrial planets. The asteroids, which are found between the two groups of planets, are believed to be remaining examples of planetesimals, the pre-planetary bodies from which the terrestrial planets formed. Meteorites are samples of asteroids that have arrived on the Earth, by a mechanism discussed in Section 1.9, and so provide direct evidence of the overall compositions of the terrestrial planets.

We probably learn more about the formation of planets from their differences than from their similarities. Several features of the Earth distinguish it from all other bodies in the Solar System and require special explanations.

1. (i) The Earth is the only planet with abundant surface water, both liquid and solid.

2. (ii) It is also the only planet with an atmosphere rich in oxygen.

3. […]