The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

Now you must bear in mind that the huge rotating mass, as imagined by LaPlace, was very many times larger than the size of our present sun. Indeed, instead of having a diameter of only 866,500 miles, its temperature was so high that the nebulous matter of which it was composed had expanded it so much that it extended far beyond the orbit of Neptune, or had a diameter twice as great as 2,766,000,000 miles.

As the huge mass continued to shrink or contract, its rotation began to gradually increase until at last its centrifugal force was sufficiently great to cause it to bulge out at the equator, so as at last to separate a ring of gaseous matter. This ring was left behind by the sun, as it continued cooling, and formed the first planet that was born into the solar system. The ring might have continued to revolve around the sun for a time, and would, of course, revolve in the same direction as that in which the sun was rotating, that is, from west to east. Eventually, however, it broke up into smaller fragments, that afterwards collected in a single body, and, assuming a globe-like shape of the planet, formed the planet Neptune. Necessarily, too, the planet so formed not only would revolve in its orbit from west to east in the same direction in which the sun was revolving on its axis, but would also rotate or spin on its axis in the same direction.

After, in this way, throwing off the first planet, the central sun continued to cool and grow smaller, until the increase in the rate of its rotation was again such as to permit its centrifugal force to form a second ring around its equator, which being left as the sun continued to contract, gave rise to another planet, or to Uranus, and so on until the four major planets and the four minor planets were born.

According to this hypothesis, the planet that was first born was the planet that is farthest from the sun, that is, Neptune, and the planet last born must have been the nearest planet, Mercury.

But while all this planet forming was going on, the separate planets also continued to shrink, and, therefore, began to rotate more rapidly on their axes. Under the influence of the centrifugal force, ring-like masses began to form around their equators, and these masses left by the planet constituted their moons or satellites. As you can see, according to this hypothesis, just as the planets would all revolve in their orbits from west to east, and rotate on their axes in the same direction as the sun, so, too, the moons or satellites of the planets would also rotate on their axes, from east to west, and revolve in their orbits in the same direction.

In order to show the extent to which LaPlace's nebular hypothesis explains the peculiarities of the solar system, we must inquire what are the most important of these peculiarities. We will take these from Young's general book on Astronomy, from which most of the facts in this chapter have been condensed. They are as follows:

The orbits of nearly all the planets and their satellites are nearly circular; they are all in the same plane; and all revolve in the same direction. They are, moreover, with the single exception of Neptune, arranged at distances from the sun in accordance with Bode's Law.

All the planets increase in both directions, towards and from the sun, in density from Saturn, the least dense.

All the planets, with the exception probably of Uranus, rotate in a plane that is nearly the same as the plane of the orbit in which they revolve. Moreover, with the exception of probably both Uranus and Neptune, all the planets rotate in the same direction as that in which they revolve.

The satellites revolve in orbits whose planes nearly coincide with the plane of the planets' rotation, while the direction of the revolution of the satellites is the same as that in which their planets revolve.