Introduction

The formation of our solar system followed the collapse and fragmentation of a dense interstellar molecular cloud. As interstellar matter always has some angular momentum, the development of a central star by direct infall was not possible, and instead a rotating disk resulted. Material within the disk lost angular momentum through viscous dissipation or other processes, leading ultimately to the growth of a central star, our Sun. Only a tiny fraction of the mass of the solar system (∼0.1%) was left behind in the disk, eventually to form the planets and asteroids.

The duration of the initial collapse phase was short, less than 1 million years. After this phase, the remnants of the accretion disk may have persisted for as long as 10 million years before the planets were assembled. This history derives from astronomical observations and is consistent with isotopic evidence from meteorites (Podosek and Cassen, 1994). The mixture of gas and grains that made up the proto-solar accretion disk is known as the solar nebula.