THE ORIGIN OF THE SOLAR SYSTEM

Men in every age have speculated as to the constitution and origin of our world, and of the Universe in general. The first really detailed and scientific attempt was made, however, little more than a hundred years ago by Laplace—and subsequently known as the Laplacian hypothesis (1796).

Concurrent with the establishment of new facts, there was a tendency, throughout the past century, to find some philosophic interpretation of the Universe and its structure; to ascertain, if possible, the “beginnings of things,” and explain them in some satisfactory manner. This has been considered as epoch-making in astronomical research as Darwin’s great theory of the Origin of Species was in biology. The history of the two theories has been similar also. Both have served a useful purpose; have helped to direct scientific thought for years; and both are now largely outgrown. Both were, however, of great value and of daring originality.

Laplace assumed the primal existence of a glowing ball of gas rapidly revolving about an imaginary axis running through its center of gravity. During the process of cooling, this mass would contract, and a disk of gas would be thrown off in this manner; and hence a number of gaseous rings be formed, which would ultimately cool down and assume a spherical form. Laplace conceived that this process might be interfered with by internal accident and by comets from without.

The first modifications of the theory were suggested by Sir Norman Lockyer, who proposed what is known as the meteoritic hypothesis in its place. The central idea of the theory was that—“All self-luminous bodies in celestial space are composed either of swarms of meteorites or of masses of meteoric vapor produced by heat.” The theory was based on spectroscopic analysis. It said that the original nebulæ were composed, not of gases, but of meteoric material and cosmic dust. This theory was never fully accepted in place of that of Laplace, however; but it paved the way for a more recent theory, which may be said to be satisfactory and more or less inclusive. This is known as the planetesimal hypothesis, and was advanced within the past few years by F. R. Moulton and T. C. Chamberlin, of the University of Chicago. At the present time, it may be said to be the accepted theory, so far as any such theories are accepted, since it accords with all the facts in a remarkable manner, and has been experimentally demonstrated. In outline, the theory is as follows.

If examination of the nebulæ in the sky be made, out of 120,000 of them, nearly every one of them is found to be in the spiral form. So common and universal is this, indeed, that it was concluded that this must represent “some prevalent process in celestial dynamics.” This process is, according to Chamberlin, the actual formation of a solar system. As this spiral revolves, it accretes to itself various smaller bodies, with their gases, atmospheres, etc., and these become consolidated with the original body. As time went on, this spiral gradually tended to decrease its speed, but at the same time, continued to accrete bodies which came into contact with it in its flight through space. Thus, we have to imagine our world, not as an expanded molten mass which has continuously cooled and contracted, but, on the contrary, as a small lump of cold and solid fragments that, moving about in accordance with its attractions, continuously fed upon its surrounding assemblage of “smaller fry,” and thus grew to its present size. About the young earth so engaged it is possible to read, on the basis of the hypothesis, something of its early history.

Thus we see that the old theory of Laplace has been reversed; and that, instead of a great central mass of moving, white-hot gas, we have a number of smaller bodies, all busily engaged in building up themselves, at the expense of the surrounding masses of still smaller matter—much as a crystal accretes to itself minute specks of crystalline matter from the solution in which it is immersed. This is the newest of the cosmological theories. According to it, all the planets might have been formed at the same time. This view of the formation of the universe opens up still wider problems, which are now the subject of keen debate.