Extraterrestrial Civilizations - Isaac Asimov [50]
NEBULAR HYPOTHESIS AGAIN
During the early 1940s, with the nebular hypothesis long dead and the catastrophic theory freshly killed, there was the uneasy feeling that no theories would explain the existence of the Solar system. It almost seemed that in sheer desperation one would have to believe that the Solar system was created by divine intervention after all, or that it didn’t exist.
In 1944, however, the German astronomer Carl Friedrich von Weizsäcker (1912–) returned to a form of the nebular hypothesis and introduced into it the kind of refinements that the developing state of knowledge had made possible since Laplace’s day a century and a half before.* According to the new version, the Sun did not contract and give off rings of gas in the process. Instead, the original nebula contracted, but left gas and dust behind as it did so. In this gas and dust, turbulences were set up—large whirlpools so to speak.
Where these whirlpools met, the particles in them collided and formed larger particles. At the very outskirts of the original nebula such particle formation may have resulted in a vast belt of small icy bodies, a few of which, now and then, alter their orbits under the influence of the gravitational attraction of nearby stars and enter the inner Solar system. There they make their appearance to us as comets.†
Closer to the Sun, where the clouds of dust and gas are denser and more massive, larger bodies are formed—the planets.
The exact mechanism whereby the planets grew out of the turbulences wasn’t easy to work out. Astronomers such as Kuiper and chemists such as the American Harold Clayton Urey (1893–) improved on Weizsäcker’s notions and suggested methods that apparently would allow the planets to grow satisfactorily.
There is still the matter of angular momentum, though. Why does the Sun turn so slowly that almost all the angular momentum is contained in the planets? What slowed the Sun?
Laplace understood the workings of gravitation, of course; no one better in his time, and few better since. In Laplace’s time, however, there was no real understanding of the electromagnetic fields that stars and planets also possess. Astronomers now know a great deal more about them, and these fields can be taken into account in any description of the origin of the Solar system.
The Swedish astronomer Hannes Olof Gösta Alfven (1908–) worked out a detailed description of the manner in which the Sun gave off material in its early days (like the Solar wind of today, but stronger) and how this material, under the influence of the Sun’s electromagnetic field, picked up angular momentum. It was the electromagnetic field that transferred angular momentum from the Sun to material outside the Sun and made it possible for the planets to be as far from the Sun as they are and to possess as much angular momentum as they do.
Now, a third of a century from the return of the nebular hypothesis, astronomers accept it with considerable confidence, along with its consequences.
In the new version of the nebular hypothesis, the outer planets are not older than the inner planets; all the planets and the Sun itself are of the same age.
Furthermore, if the Sun and the planets formed out of the same whirlpools of dust and gas, all developing in the same process, then this is very likely the way in which any star like the Sun (and just possibly any star at all) develops. There should, in that case, be very many planetary systems in the Universe and just possibly as many planetary systems as there are stars.
THE ROTATING STARS
Is there any way we can check this suggestion of the universality of planetary systems? Theories are all very well, but if there is any physical evidence that can be gathered, however tenuous, so much the better.
Suppose we had evidence to show that planetary systems were few. We would have to suppose the Weizsäcker theory of star formation was wrong, or at least that it must be seriously modified. Perhaps the Sun formed in lonely splendor, and then passed through another cloud of dust and gas in space