The Context of Evolution

Abstract

Our Solar System shows very little evidence of having been at peace with itself since its formation. In line with a hypothesis put forward in 1796 by the French physicist Pierre Simon, Marquis de Laplace (1749–1827), following an earlier suggestion by the German philosopher Immanuel Kant (1724–1804), its origins are generally thought to have involved the condensation under gravity of a large cloud of dust and gas, some 4600 Myr (million years) ago. However, it now seems likely that the first product of such a gravitational collapse was a spinning disc of dust particles, rather than the Solar System as we know it today. The proto-Sun formed at the center, while around it a process of accretion began to take place. Collisions between dust grains led, very slowly, to the formation of pebbles, after which further collisions gave rise to boulders and later to planetesimals. The situation must have been highly complex, with bodies travelling in all manner of orbits as a result of fleeting encounters with other bodies. Head-on collisions caused fragmentation, whereas bodies in similar orbits that came together gently were the ones most likely to merge. Once planetesimals larger than a few kilometers in diameter came into existence, the process of accretion would have started to accelerate. [1–10] However, that cannot be the end of the story, for had the planets, moons, and other large bodies so formed then remained in orbits that did not bring them into conflict with each other, the Solar System today would surely show more consistency of features, with far fewer anomalies, than it actually does.