Extraterrestrial Civilizations - Isaac Asimov [48]
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.
STELLAR COLLISIONS
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.* 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