What is the relationship between star formation in nebulae, and planet formation?
First of all, the scales involved are so vastly different that even astronomers do not know for certain exactly how planet formation follows from the formation of individual stars. We can see dense cores of gas embedded within some nebulae in interstellar space, and deduce from the appearance of certain spectral lines that these cores are undergoing collapse, but even so, these cores are several light years across, and the density of the gas is about 1000 to 10,000 atoms/cc. This is a long way from the scales and densities we see in planets in our solar system which are measured in a few thousand kilometers, and upwards of 10^20 atoms/cc. It is generally believed that star formation is a very messy business at scales of the order of our solar system or smaller. Orbiting accretion disks or ‘proto-planetary’ disks seem to be a very strongly favored dynamical system for the matter that has not fallen into the growing proto-star. Astronomers have now seen dozens of these circumstellar disc
