Extraterrestrial Civilizations - Isaac Asimov [49]
Close analysis of the catastrophic theory, however, suggested difficulties. Could the streams of matter issuing from the Sun extend so far outward as to give rise to the outer planets? Could the gravitational influence of the other star transfer enough angular momentum to the planets?
As a result, astronomer after astronomer attempted to modify the theory to make it more plausible. Some suggested an actual grazing collision rather than a mere passby. The American astronomer Henry Norris Russell (1877–1947) suggested that the Sun had been part of a two-star system, with the planets born of the other star so that they possessed its momentum.
Despite the difficulties, the catastrophic theories reigned supreme even into the 1930s, and this was a matter of crucial interest with respect to the thesis of extraterrestrial intelligence.
If the nebular hypothesis or any evolutionary theory of the Solar system were correct, then planets were formed as part of the normal development of a star and there were, essentially, as many planetary systems as there were stars. In that case, the chances of extraterrestrial intelligence might be very good.
The catastrophic theories, on the other hand, made planetary formation an accidental and not an inevitable thing. It depended on a sort of cosmic rape, on the fortuitous coming together of two stars.
As it happens, stars are so widely separated and move so slowly in comparison with the distance of separation that the chances of such a collision or near-collision are exceedingly small. During its entire lifetime, a star like the Sun has only one chance in 5 billion of closely approaching another star. In the entire lifetime of the Galaxy, there may have been only fifteen such close approaches outside the Galactic nucleus.
If any form of the catastrophic theory should correspond to reality, it would mean that there are very few planetary systems in the Galaxy, and the chance that any one of those few should harbor a civilization (excluding our own, of course) would be extraordinarily small.
Fortunately for the chances of extraterrestrial intelligence, however, the catastrophic theories proved less tenable with each decade.
Despite all the modifications introduced, there remained great difficulty in giving the planets sufficient angular momentum. Any mechanism that could be devised to provide it was all too apt to give them enough speed to cause them to escape from the Solar system altogether.
Then, in the 1920s, the English astronomer Arthur Stanley Eddington (1882–1944) worked out the internal temperature of the Sun (and of stars generally). The Sun’s enormous gravitational field tends to compress its matter and pull it inward, yet the Sun is gaseous throughout and has a density only about a quarter that of the Earth. Why does it not condense to much greater densities under the inexorable inward pull of gravity?
To Eddington, it seemed that the only thing that could counteract the inward pull of gravity would be the outward expansive force of internal heat. Eddington calculated the temperatures required to balance the gravitational inpull and showed, quite convincingly, that the Sun’s core had to be at temperatures of millions of degrees.
If then, as a result of a collision, or near collision, large amounts of matter were pulled out of the Sun, or of any star, that matter was going to be at much higher temperatures than had been thought. They would be so hot, the American astronomer Lyman Spitzer, Jr. (1914–) pointed out in 1939, that there was no chance at all they would condense into planets. They would expand into thin gas and