15.5 The Angular Momentum Problem

A possible weak link in the condensation theory is sometimes known as the angular momentum problem. Although our Sun contains about 1000 times more mass than all the planets combined, it possesses a mere 0.3 percent of the total angular momentum of the solar system. Jupiter, for example, has a lot more angular momentum than does our Sun—in fact, about 60 percent of the solar system's angular momentum. All told, the four jovian planets account for well over 99 percent of the total angular momentum of the solar system. By comparison, the lighter (and closer) terrestrial planets have negligible angular momentum.

The problem here is that all mathematical models predict that the Sun should have been spinning very rapidly during the earliest epochs of the solar system and should command most of the solar system's angular momentum, basically because it contains most of the mass. However, as we have just seen, the reverse is true. Indeed, if all the planets' orbital angular momentum were transferred to the Sun, it would spin on its axis about 100 times as fast as it does at present.

Many researchers speculate that the solar wind, moving away from the Sun into interplanetary space, carried away much of the Sun's initial angular momentum. The early Sun probably produced more of a dense solar gale than the relatively gentle "breezes" now measured by our spacecraft. High-velocity particles leaving the Sun followed the solar magnetic field lines. As the rotating magnetic field of the Sun tried to drag those particles around with it they acted as a brake on the Sun's spin. Although each particle boiled off the Sun carries only a tiny amount of the Sun's angular momentum with it, over the course of nearly 5 billion years the vast numbers of escaping particles could have robbed the Sun of most of its initial spin. Even today, our Sun's rotation rate continues to slow.

Other researchers prefer to solve the Sun's momentum problem by assuming that the primitive solar system was much more massive than the present-day system. They argue that the accretion process was not entirely successful during the system's formative stages. Matter not captured by the Sun or the planets might well have transported much angular momentum back into interstellar space as it escaped. This proposal is difficult to test, because the escaped matter would be well beyond the range of our current space probes. Perhaps the remote Oort cloud of innumerable comets is the "escaped" matter.

Despite some minor controversy as to how this angular momentum quandary can best be resolved, nearly all astronomers agree that some version of the condensation theory is correct. The details have yet to be fully worked out, but the broad outlines of the processes involved are quite firmly established. Our planet is a by-product of the formation of the Sun. We might reasonably now ask what preceded the Sun, and what circumstances led to the collapse of the solar nebula in the first place. To answer these questions, we must widen the scope of our studies. We will find it necessary to understand the workings not only of the stars and the gas between them but also of the Galaxy in which they reside.