The Ways of the Planets

by Martha Evans Martin

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.