Extraterrestrial Civilizations - Isaac Asimov [47]
If the Sun had formed out of the condensation of a spinning cloud of dust and gas (and we can see many such clouds in our Galaxy and in some other galaxies as well), it is reasonable to suppose that other stars formed in the same way.
Since our Sun, as it condensed, could be pictured as spinning faster and faster and losing rings of material from its equatorial region—one ring after another—thus forming the planets, other stars as they formed would do the same.
In that case, every star would have a planetary system.
We could not, however, come to that conclusion on the basis of the nebular hypothesis unless that theory of planetary formation could withstand close examination, and it didn’t.
In 1857, Maxwell (who later worked out the kinetic theory of gases) was interested in reasoning out the constitution of Saturn’s rings. He showed that if the rings were solid structures (as they seemed to be in the telescope) they would be broken up under the influence of Saturn’s gravitational pull. It seemed, therefore, that they must consist of a large aggregate of relatively small particles, so thickly strewn as to seem solid when viewed from a great distance.
Maxwell’s mathematical analysis turned out to be applicable to the ring of dust and gas supposedly shaken loose by the contracting nebula on its way to condensation into the Sun. It turned out that if Maxwell’s mathematics was correct, it was difficult to see how such a ring would condense into a planet. It would at best form an asteroid belt.
An even more serious objection arose out of a consideration of angular momentum, which is the measure of the turning tendency of any isolated body or system of bodies.
Angular momentum depends on two things: the speed of each particle of matter as it rotates about an axis, or revolves about some distant body, or both; and the distance of each particle of matter from the center of rotation. The total angular momentum of an isolated body can’t vary in quantity, no matter what changes take place in the system. That is called the law of conservation of angular momentum. By this law, the velocity of spin must increase to make up for any decrease in distance, and vice versa.
A figure skater demonstrates the principle when she or he begins spinning with the arms outstretched, and then draws those arms in. At this condensation of the human body, so to speak, the rate of spin rapidly increases, and if the arms are then outstretched, it as rapidly slows down again.
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