Extraterrestrial Civilizations (15 page)

When the rotating nebula gives off a ring of matter, this ring of matter cannot be more than a very small portion of the whole nebula. (This is obvious, since the ring condenses into a planet that is much smaller than the Sun.) Each bit of matter in the ring contains more angular momentum than a similar bit of matter from the main body of the nebula, because the ring comes off the equatorial belt where both the velocity of spin and the distance from the axis of rotation are highest. Nevertheless, the
total
angular momentum of the ring must be only a tiny fraction of the total angular momentum of all the rest of the vast nebula.

One would expect therefore that the Sun today, even after it has given off the matter required to form all the planets, would still retain much of the angular momentum of the original nebula. Its rate of spin should have accelerated so much as it shrank that it should today be rotating on its axis with violent speed.

Yet it doesn’t. A point on the Sun’s equator takes no less than 26 days to move once around the Sun’s axis. Points north and south of the equator take even longer. This means that the Sun contains a surprisingly small amount of angular momentum.

The Sun, in fact, which contains 99.8 percent of all the mass in the Solar system, possesses only 2 percent of the angular momentum in the system. All the rest of the angular momentum is contained in the various small bodies that turn on their axes and swing around the Sun.

Fully 60 percent of all the angular momentum in the Solar
system is possessed by Jupiter and another 25 percent by Saturn. The two planets together, with only 1/800 of the mass of the Sun, possess 40 times as much angular momentum.

If all the spinning, revolving worlds of the Solar system were somehow to spiral into the Sun and add their angular momentum to the Sun’s (as they would have to by the law of conservation of angular momentum), the Sun would spin on its axis in half a day.

There seemed no way in which so much angular momentum could be concentrated into the tiny rings peeling off the equatorial region of the spinning nebula and taken away from the nebula itself. Once this matter of angular momentum was clearly realized in the latter decades of the nineteenth century, the nebular hypothesis seemed to have received a death blow.

In the search for some explanation of the origin of the Solar system that would account for the peculiar distribution of angular momentum, astronomers veered away from evolutionary theories of planetary formation—that is, theories postulating slow but inexorable changes. They turned instead to catastrophic theories in which planets are formed by a sudden change that is not an inevitable part of the development, but an unexpected one.

In such theories, the original rotating nebula condenses smoothly to the Sun with no formation of planets. Rolling through space in solitary splendor, however, the Sun encounters a catastrophe that forms the planets
and
transfers angular momentum to them.

The first catastrophic theory was actually advanced in 1745, 10 years before Kant had advanced the first version of the nebular hypothesis.
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It was advanced by the French naturalist Georges Louis Leclerc de Buffon (1707–1788).

Buffon suggested that the planets, including Earth, had come into existence some 75,000 years before, as a result of a collision
between the Sun and another large body, which he called a comet. (It was a time when the nature of comets was as yet unknown, but in which they were known to approach the Sun unusually closely.) Life, he thought, had then begun 35,000 years after Earth’s formation. This conflicted with the general belief that God had created both the Earth and life less than 6,000 years earlier.

Buffon’s notion, which lacked detail, receded into the background in view of the popularity of the nebular hypothesis. By 1880, however, when the nebular hypothesis was running into trouble over the matter of angular momentum, the catastrophe notion was revived.

The English astronomer Alexander William Bickerton (1842–1929) suggested that the Sun and another star passed close by each other. The gravitational influence of each body on the other pulled a stream of matter outward. As the stars separated, the gravitational influence between them pulled that stream of matter sideways, imparting “English” to it and giving it a great deal of angular momentum at the expense of the main portion of the bodies. From the streams of matter pulled out in the near-collision, the planets formed. Two solitary stars entered the state of near-collision; two stars with planetary systems emerged. It was a dramatic picture.

By 1880, a number of the galaxies had been made out in the telescopes of the time, and many of them had a glowing nucleus, together with spiral structures outside that nucleus. This was first noted in 1845 by the Irish astronomer William Parsons, Earl of Rosse, (1800–1867).

At the time, it was not understood that these “spiral nebulae” were vast and distant assemblages of stars and that our own Galaxy was one. They were thought to be small formations within our Galaxy, and Bickerton thought that they might represent planetary systems in the process of formation, with the spiral arms representing the streams of matter pulled out of the central sun and given a strong curve that started them on their revolutions.

For the next fifty years, the catastrophic theory of planetary formation was popular with astronomers. The English astronomer James Hopwood Jeans (1877–1946) suggested that the stream of matter pulled out from the Sun was cigar shaped and that Jupiter and Saturn were formed from the fattest part of the stream, and that
that was why they were so large. Jeans was a superb writer of popular science and his influence did more to impress the general public with this theory of the formation of the Solar system than did anything else.

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 be gone.

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.
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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.

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 (there are plenty of such clouds) and collected some of it gravitationally. In that case, turbulences in the second cloud might finally form the planets, which would be younger than the Sun, perhaps a great deal younger.

This would be a return to a form of catastrophism, even though
the passing of the Sun through a cloud of gas is not nearly so violent an event as the collision or near collision of two stars. It is still an accidental event and would necessarily result in relatively few planetary systems.

On the other hand, if it turned out that the evidence clearly indicated that a great many stars happened to have planets, then we could not possibly expect this to happen in any catastrophic way. Some version of the nebular hypothesis with the automatic or near-automatic formation of planets along with a star would have to be correct.