Origin of the Solar System

Abstract

Some 4.56 billion years ago a cloud of interstellar gas and dust, perhaps triggered by a nearby supernova explosion, began to collapse. Its central part contracted under its own gravity and started to heat up, until the temperature became so high that thermonuclear reactions were initiated, and the early Sun began to release growing amounts of energy. But the peripheral part of the cloud, owing to conservation of angular momentum, flattened to a disc with a maximum density on its median plane. A fraction of this material gradually solidified and gave rise to a swarm of small lumps of matter gravitationally bound to the Sun. These bodies underwent a process of further accumulation and coalescence which, eventually, through a complex sequence of mutual interactions and disturbances, both gravitational and non-gravitational, produced planets, satellites, asteroids and comets; indeed, this is the variety of bodies which we presently observe in the solar system. In the outer part of the nebula the biggest bodies as well were able to accrete a substantial amount of gas and undergo a partial collapse, generating their own nebula, in which a similar process of coalescence and accretion produced their satellite systems. In this chapter we discuss, mainly with estimates and results of numerical simulations, several features of this process, as presently understood on the basis of a growing body of evidence. While the basic ideas of this so called nebular theory go back to I. Kant and P.S. Laplace, its quantitative contemporary version was developed in the 1950s and the 1960s by O.Yu. Shmidt, V.S. Safronov and collaborators.