Presolar Cloud Collapse and the Formation and Early Evolution of the Solar Nebula

We review our current understanding of the formation and early evolution of the solar nebula, the protoplanetary disk from which the solar system formed. Astronomical understanding of the collapse of dense molecular cloud cores to form protostars is relatively advanced, compared to our understanding of the formation processes of planetary systems. Examples exist of nearly all the phases of protostellar evolution, guiding and validating theoretical models of the star formation process. Astronomical observations of suspected protoplanetary disks are beginning to provide key insights into the likely conditions within the solar nebula, although usually only on much larger scales than the regions where the terrestrial and giant planets formed. The primary mechanism for driving the early evolution of the solar nebula, leading finally to the formation of the planetary system, is still highly uncertain: Magnetic fields, gravitational torques, and baroclinic instabilities are among the competing mechanisms. While the discovery of extrasolar gas giant planets has reinforced the general belief that planetary system formation should be a widespread process, the absence of any undisputed evidence for giant protoplanets in the process of formation makes it hard to decide between competing mechanisms for their formation. The situation regarding extrasolar terrestrial planets is even less constrained observationally, although this situation should improve tremendously in the next decade.