Origin of the Solar System: Evolution of the Earth and of Life

Abstract

From the depths of cosmic space we now return to our planetary system with the old question of its origin. So long ago as 1644, in France René Descartes with his vortex theory was able to advance the daring idea that one could come nearer to the answer, not by handing on traditional myths, but by proper investigation. In Germany even in 1755 I. Kant had to let the first edition of his Allgemeine Naturgeschichte and Theorie des Himmels appear anonymously because he feared the (protestant) theologians. In the book he treated the formation of the planetary system for the first time “nach Newtonischen Grundsätzen”. Kant started from a rotating, flattened primeval nebula out of which the planets and later the satellites were then formed. A similar hypothesis was at the basis of the somewhat later (independent) account by S. Laplace 1796 in his popular book “Exposition du Système du Monde”. We shall not go into details and differences of these historically important beginnings, but we shall summarize once more the most important facts (see Table 7.1 and Fig. 31.1) which have to be explained:—

• 1. The orbits of the planets are almost circular and coplanar. Their sense of revolution is the same (direct) and agrees with the sense of rotation of the Sun. The orbital radii (the asteroids are taken together) form approximately a geometrical progression

  $$ {a_n} = {a_0}{k^n} $$

  (31.1)

  where a _o =1 AU, with n = 0 for the Earth, and k ≈ 1.85 (Fig. 31.1).

• 2. The majority of satellites move in orbits of small eccentricity near to the equatorial planes of their planets. Also the inclinations of these equatorial planes to the ecliptic or, to put it better, to the invariable plane of the planetary system (perpendicular to the resultant angular-momentum vector) are mostly small. The rotation of the planets and the revolution of their satellites proceed for the most part in the direct sense. Exceptions (Pluto and certain satellites) occur mainly near the edge of the system concerned.

• 3. The terrestrial planets have relatively high densities, the major planets have low densities (Table 7.1 and Fig. 31.1); the former consist (like the Earth) in the main of metals and rocks, the latter of scarcely modified solar material (hydrogen, helium, hydrides). The terrestrial planets have slow rotation and few satellites; the major planets have relatively rapid rotation and numerous satellites.

• 4. The Sun retains within itself 99.87 percent of the mass but only 0.54 percent of the angular momentum (Σ m r y) of the whole system, while conversely the planets (mainly Jupiter and Saturn) have only 0.135 percent of the mass and 99.46 percent of the angular momentum *.