The environment and physics of the early solar system

Abstract

Astronomical observations and modelling suggest that stars assemble out of the gas and dust collected in the cores of vast clouds in interstellar space. These cores gravitationally attract more material from the surrounding cloud, until enough mass is concentrated to trigger their gravitational collapse. The result of this process is a growing object (a protostar), slowly fed by a flattened disc of material (aprotostellar disc) that encircles it. These discs are the analogues of the early solar system, and potential sites of current planet formation. Provided that disc material can shed its excess angular momentum, matter will flow onto the central object, and it is believed that this is how stars typically gain most of their mass. Although this basic paradigm appears solid, many aspects of this process remain unexplained. The mechanisms responsible for this transport of angular momentum are not well understood. The most promising are turbulent motions driven by the magnetorotational instability, and large-scale outflows accelerated from the disc surfaces. Both processes are driven by magnetic fields and are, in turn, likely to strongly affect the structure, evolution and planet-forming processes in the disc. The low ionisation of the material, however, may prevent the magnetic field from driving these processes in some regions of the disc. On the other hand, a record of the formation process of our own solar system is preserved in primitive meteorites, the ‘building blocks’ of the solar nebula. Preserved in the samples are a variety of objects that have experienced very high temperatures (∼1700–2000 K), evidence that could imply a hot ambient temperature in the solar nebula. In contrast, the preservation of presolar grains in meteorites, geochemical evidence for a poorly mixed nebula, and astrophysical observations of forming stars suggest a cool protoplanetary disc, where temperatures are too low to produce this thermal processing. The coexistence of hot and cold material in the early solar system has remained unexplained for many decades. Magnetic fields, again, may hold the key to solve this longstanding puzzle in planetary science. In this review I first examine the meteoritic record and key properties of protostellar discs. I then discuss the nature and viability of the processes thought to enable accretion in these systems, focussing on the dependence of these processes on the disc environmental conditions. The implications for planet formation are also discussed.