The Formation of the Solar System

Abstract

Although the observation of objects in the Solar System has been practiced, to a considerable degree of accuracy, since ancient times, the problem of its origin was not really considered until after the Copernican revolution. This, which merely repeated, twenty centuries later, the theory first advanced by Aristarchos of Samos, located the Sun at the centre of the system. The first models for the system’s formation tried initially to explain, more or less in qualitative terms, the observed movements. Various factors had to be taken into account: (1) the orbits of the planets are close to the plane of the Earth’s orbit; (2) the orbits are essentially circular (with the exception of those of Pluto and Mercury); (3) the planets all rotate in the same sense, which is the same as that of the Sun. These particular constraints applied to all theories developed up to the 19th century. At the end of the 19th century, and the beginning of the 20th, theoreticians started to pay attention to the problem of angular momentum. The Sun, which contains 99.8 per cent of the mass of the Solar System, only has 2 per cent of the total angular momentum possessed by the planets. Finally, the second half of the 20th century benefited from the contribution of new theories concerning stellar formation, as well as new data concerning the dating and composition of the various bodies. All these new elements have allowed us to pick out a coherent model for the formation of the Solar System, even though all the various physical and chemical mechanisms are still not fully understood.