Solar Nebular Theories

Over the centuries there have been several different types of theory on the origin of the Solar System, but in recent decades one type, with antecedents in the eighteenth century, has emerged as the firm favourite. This is the solar nebular theory. Theories of this type are characterised by the formation of the planetary system from a disc of gas and dust encircling the young Sun - the solar nebula. This is clearly in accord with the relatively recent observations of circumstellar discs around protostars and young stars. Overall, such theories fit the observational data better than any other type of theory, and there are certainly no observations that rule them out. Within the general type there have been many variants, though there has been some degree of convergence so that most variants now differ only in relatively minor details. We shall concentrate on the typical features of solar nebular theories, pointing out where the variants differ significantly.

We pick up the story at the point where the proto-Sun is surrounded by a disc of gas and dust of order 100 AU across - the solar nebula. This is shown edge on in Figure 2.3(a). The disc would not have ended as abruptly as shown; it is the extent of the main bulk of the disc that is indicated. The plane of the disc coincides with the equatorial plane of the proto-Sun, and the disc and proto-Sun, being derived from a single dense cloud fragment, will rotate in the same direction. The disc rotates differentially, the orbital period increasing with distance from the centre, in accord with Kepler's third law. The elemental composition of the disc is much the same as the present Sun outside its core - we have adopted, by mass, 70.9% hydrogen, 27.5% helium, 1.6% the rest. The gas in the disc is predominantly hydrogen and helium, and a significant fraction of the other elements is in the various compounds that constitute the dust. The formation of planets in this disc will lead to their orbiting in the same plane, all in the same direction, the direction of solar rotation. These are features of the Solar System that any acceptable theory of its origin has to explain (Table 2.1).

Rotation axis

~105 years after (a)

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