Successes and Shortcomings of Solar Nebular Theories

The solar nebular theory outlined in this chapter accounts for many of the features in Table 2.1. It accounts for other features too. Overall, it is a successful theory. Perhaps the most worrying aspect left unexplained is the 7.2° tilt of the solar rotation axis with respect to the ecliptic plane. Whereas it is not difficult to account for the axial tilts of many of the planets' axes by the effect of material added asymmetrically, it is less easy to understand how the addition of material to the proto-Sun could have been sufficiently asymmetrical. A possible explanation is that there was a close encounter between the proto-Sun and another young star in the cluster in which the Sun was born.

As with any scientific model, the nebular theory is not fully explored. Perhaps the most important area that needs further exploration is the timing of the T Tauri phase with respect to the evolution of the nebula. This is crucial to the final configuration of a planetary system. □ Why is this?

The T Tauri phase clears gas from the nebula, and thus halts the growth of the giant planets (Section 2.2.5).

Nevertheless, the great majority of astronomers believe that solar nebular theories are in fairly good shape, and offer by far the best type of theory that we have for the origin of the Solar System.

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