Different as the planets are as individuals, they have too many characteristics in common to admit any question of their common origin. They are not simply stars of one sort and another that happen to lie nearer to us than the great body of stars that spangle the heavens, but are, without doubt, all of one family with us in their origin, as well as in their situation. How they originated, and exactly what has been their course of evolution, has long been an engrossing problem among philosophers; and it is not yet solved.

In the sense that the human race is all of one family, the planets are but a part of the great universe that lies about us and is in part visible to us. The forms in which we know matter as existing in the universe, outside of the solar system and of the minor forms in our own world, are those of stars and nebulÆ. It seems as if either of these could, and in fact does, form out of the other. We do not at all know how in the beginning matter took the form of either, or which came first. But it is believed that a star is formed by the condensation of a nebula, and that a nebula is often formed by the collision or near approach of two stars and the consequent disintegration of their particles.

The sun is a star not very different from most of the other stars, as we believe them to be, except that it is smaller than most of them. It is the center around which we and all the planets revolve, and it is believed that we were all once a part of the very body of it. For astronomers are substantially agreed that the whole solar family, including the sun and all the planets, has been evolved from a great nebula which, in one form or another, at one time filled practically the whole of the immense space from the sun to the outermost planet of the system. While this cannot be said to have been exactly proved, yet it accords with all the known facts of the solar system. As to how this nebula originated, and what its shape was, and in just what way the planets were formed from it, there is more diversity of opinion.

Up to the middle of the eighteenth century no really scientific theory of the evolution of the solar system was formulated, and it was not until the very last years of that century that any theory of the origin of the planets was published which received anything like universal acceptance.

This was the case, however, with the famous nebular hypothesis of Laplace, which was published in 1796, and for a time seemed so nearly to account for the various phenomena of the motions and relations of the planets that it was not only accepted in the scientific world, but became almost as much a part of universal knowledge as that the earth is round. But even this theory has not completely stood the test of time, which inevitably brings that close scientific investigation that any theory must undergo when it is used as a working basis to which all facts and secondary theories must be correlated.

The original nebular hypothesis supposed this vast nebula to be in rotation on its axis. As it condensed, the falling-in of the particles caused its rotation to become more rapid, until finally, under the strain of this, a ring of matter was “thrown off” from the outer edge. Or, as was sometimes said, the inner part condensed and left a detached ring of matter. This ring, continuing to rotate in the direction given it by the rotation of the central mass, finally condensed into a planet, rotating on its axis and revolving about the central sun in the same direction as the ring had revolved. The satellites of the planets were thought to have been formed by the same process from the planets while these were still in a plastic state. Saturn, with its wonderful system of rings and satellites, was thought to be a minute object-lesson of a planet in course of evolution, and this we have often heard said.

I am sorry it is not so. I had much enthusiasm in my youth over this beautiful and orderly arrangement of things: first, the splendid hypothesis, the achievement of a noble mind; then the little model showing the work in its progress; and, finally, the beautifully finished system, the rings all rolled up into planets, traveling unceasingly in paths which eternally marked the size of the central body, or sun, at the time of the separation.

But it is now pretty certain that this cannot be the way it all happened. Closer investigation shows that there are mechanical difficulties which were not at first fully recognized. A series of rings could not have been left off by a body so wholly gaseous. The particles composing them would not be sufficiently coherent to permit of separation in any such compact, uniform, and decisive manner. Then, even if such a ring were thrown off, it is not at all certain that it could condense into a planet. Its tendency, indeed, would be to disintegrate rather than to condense. In a body so tenuous the mutual gravitation of its particles would be too feeble to complete the work. Besides, in conflict with the theory is the fact that a few of the satellites of the planets revolve in a direction contrary to that of the planet. And there are other minor, but still important, details in the mechanism of the solar system which cannot be accounted for by the ring theory.

And so, while astronomers are still agreed that the whole solar system, which includes the planets, was evolved from a primeval nebula, the theory of leaving off rings which condensed into planets is not found tenable, and the search for some more acceptable theory or some modification of the Laplace theory is now occupying a number of eminent astronomers and philosophers.

The result of all this is that no theory of the manner of the evolution of the planets is definitely accepted by the body of astronomers. Much hard labor and ingenious reasoning have been expended in endeavoring to formulate some hypothesis by means of which we may account for observed phenomena. The astronomers with whom these theories have originated are, naturally, more or less ardent in setting them forth. Thus one occasionally sees a decisive and authoritative statement of a theory of the evolution of the planets that seems at first view to account for everything. But no one of these has yet been entirely accepted by astronomers, who are as a class cautious and conservative, and are necessarily critical of any theory, because the value of much of their future work depends upon its accuracy and sufficiency for all details.

The theory which at present seems more nearly than any other to offer a reasonable explanation of most planetary phenomena is based upon the supposition that the nebula from which the sun and planets were evolved was in the shape of a spiral, and not the gaseous mass that the original nebular hypothesis supposed. The fact that among the many thousands of nebulÆ that have been discovered and observed a very large proportion of them are in this form, aside from any other consideration, suggests a great probability that the one from which the solar system was evolved was a spiral.

The spiral nebulÆ seem to be of a somewhat different constitution from the other nebulÆ, and show on observation spots of condensation here and there, which at least suggest the formation of systems of planets. This indicates that ours may be only one of many such systems in process of evolution; but it is certainly among the smallest of them, for most of the spiral nebulÆ are immensely greater in size than the one required to form our little system. Its few trillions of miles of diameter, though it seems so vast to us, is quite insignificant in comparison with a large proportion of the spiral nebulÆ in the universe.

A spiral nebula is in the form of a disc somewhat resembling that familiar form of fireworks known as a pinwheel. The typical form of it has two arms projecting from opposite sides of the whirling figure. It is much denser toward the center, where the spiral would naturally be more tightly wound, and has smaller spots of condensation scattered like knots here and there along the fiery arms. In the process of evolution the denser center becomes the controlling sun, and the smaller spots of condensation form the planets, which are eventually detached from the revolving mass, but continue to revolve about the center as they were doing from the beginning. According to the mass it has in the beginning, the planet gathers up by gravitative attraction all the material in its region, gaseous or more or less condensed, and grows by this accretion. If the nucleus happened to be a large one before it separated from the parent body, it will have sufficient force of gravitation to gather in large quantities of material and greatly increase its size, and thus become a large planet. If it is only a small nucleus, it has less power of attraction, and gathers in less material.

When these condensations of matter which are the nuclei of the planets break away from the parent body, they sometimes carry with them still smaller nuclei, which, if they are not too near the original center, or sun, are destined to remain under the control of the planets and become their satellites. The number and size of the satellites a planet has depends upon the size, and hence the controlling force, of the nucleus which is its foundation, and also upon the number of spots of condensation that chanced to be formed in its neighborhood sufficiently near to come under the gravitational control of the planet. If by any chance the nucleus which was to form the largest satellite of Jupiter had been in the situation of Mercury, for instance, it might well have given its allegiance to the sun, instead of to Jupiter, and thus have become a planet.

Under the ring theory the outermost planet, Neptune, would be the oldest of the planet family, and the one nearest the sun, Mercury, would be the latest born and youngest. But the physical development of these planets seems to indicate, in truth, exactly the opposite of this, as we shall see later on. Under the spiral-nebula theory the planets may be nearly of the same age, their different states of development being due mainly to difference in size and to some peculiarities of situation. If the nucleus happened to be near the outer edge of the spiral, it would be formed from the lighter matter composing the outer part of the nebula, and this seems to be the case with the outer planets. If it were near the dense center of the nebula, it would be composed of denser material, and this seems to be so in the case of the inner planets.

A nebula, it is thought, is formed by the collision or the near approach of two of the many stars, or suns, that we know are traveling about at high velocities as vagrants here and there through space. If the two bodies come together centrally, the force of the impact will generate heat sufficient to convert them into a nebula; but this will not necessarily be spiral in form. If they come together obliquely, the chances are that they will form into a rapidly rotating spiral disc.

But in order to form a spiral, it is not necessary that there should be an actual collision. Because of the force of gravitation the near approach of two stars would subject them to an enormous strain from their pull upon each other, and there is a limit within which they cannot approach without being literally torn to pieces from the effect of this tidal force. Even if they do not approach within this fatal limit, which is a little less than two and one-half times the radius of the body, they may come so near as to change their character entirely, and, through their tidal influence on each other, form into a rotating spiral nebula with two arms projecting from opposite sides of the spiral.

It now seems probable that it was after this manner that the sun and its family of planets were formed. The matter which is contained in them may have been in the form of a dark, solid body pursuing some sort of course in space. In its journeying it came near another body and was awakened into a life of activity in the form of a flat, spiral nebula which was left spinning around in a pyrotechnic manner, the matter composing it much diffused at the outer edges and densest in the center. Scattered through it were the more or less condensed spots which were the embryonic forms destined to come forth from the parent body as the individual planets.

When the separation was completed, each planet fed and grew upon all the matter that it had the force to draw to it, and it swept clean the space that lay within the limits of its power. If the particles thus gathered in were small and slow of motion, they became a part of the body of the planet. If they were large and swift, they became members of the planet’s family as satellites. In whatever area of the nebula each planet came into a separate existence, it fed upon the matter which that area afforded. In the case of Neptune, at the outer edge of the system, it was very diffuse matter; in Mercury’s region, nearer the center, it was more dense.

Thus in our family of planets, though its members were born of the same parent and developed under the same guiding laws, each has a distinct individuality arising from its inherent qualities and its environment during the early stages of its existence. The spiral-nebula theory seems to offer a better explanation of these individual qualities than any other that has been advanced thus far, and in its main features it is pretty generally accepted. But one must keep in mind that the details of any theory of the beginning and growth of the planets are more or less speculative, or, at least, have not yet been proved with finality.