The condensation theory accounts for the nine "characteristic" points listed at the start of this chapter. Specifically, the planets' orbits are circular (2), in the same plane (3), and in the same direction as the Sun's rotation on its axis (4) as a direct consequence of the nebula's shape and rotation. The rotation of the planets (5) and the orbits of the moon systems (6) are due to the tendency of the smaller-scale eddies to inherit the nebula's overall sense of rotation. The growth of planetesimals throughout the nebula, with each protoplanet ultimately sweeping up the material near it, accounts for point (1), which is the fact that the planets are widely spaced (even if the theory does not explain the regularity of the spacing). The heating of the nebula and the Sun's ignition resulted in the observed differentiation (7), and the debris from the accretionfragmentation stage naturally accounts for the asteroids (8) and comets (9).
We stressed earlier that an important aspect of any solar system theory is its ability to allow for the possibility of imperfectionsdeviations from the otherwise well ordered scheme of things. In the condensation theory that capacity is provided by the randomness inherent in the encounters that ultimately combined the planetesimals into protoplanets. As the numbers of large bodies decreased and their masses increased, individual collisions acquired greater and greater importance. The effects of these collisions can still be seen today in many parts of the solar systemfor example, the large craters on many of the moons we have studied thus far.
Having started with nine regular points to explain, we end with eight irregular solar system features that still fall within the theory's scope. It is impossible to test any of these assertions directly, but it is reasonable to suppose that some (or even all) of the following "odd" aspects of the solar system can be explained in terms of collisions late in the formative stages of the protoplanetary system. Not all astronomers believe all these explanations; however, most would accept at least some.