The difficulties in the way of believing that the moon once formed a part of the earth are very great. It can be shown mathematically that if the two bodies at one time formed a single mass it would have been impossible for the moon to break away from the earth, unless the force that caused the separation were sufficient to hurl the moon to a greater distance than two and a half times the earth's radius. The mathematician, Roche, found out by computation that a satellite could not remain intact within this distance of the planet, but would be broken up into small fragments under the effects of the tides raised by the larger body. If, then, the moon had originally been ejected from the earth to a less distance than two and one-half radii of the earth (2.44 to be exact) it would have been disintegrated into small particles, or moonlets, under the tidal strains exerted upon it by the earth and would have been gradually distributed about the earth in the form of a meteoric ring which, in the course of ages, would be absorbed by the earth, just as Saturn is now gradually absorbing its rings.
The planets differ greatly in density. The more distant and larger planets—Jupiter, Saturn, Uranus and Neptune—have densities equal to or less than that of the sun. The densities of the inner planets—Mercury, Venus, Earth and Mars—are, relatively, extremely high, the density of the Earth's core being about that of meteoric iron. The densities of Mercury and Venus are slightly less than that of the earth and the densities of Mars and the moon about equal to that of the earth's crust.
If a stream of matter were ejected from the sun under the influence of some external force, such as that exerted by a passing star, the outlying parts of the stream would consist of the lighter elements and the lower parts of the heavier elements, since the lighter solar elements lie at or near the surface of the sun and the heavier elements at greater depths. At the time of ejection the lighter elements would be thrown to great distances and would go to form the less dense outer planets; the heavier elements would go to form the inner planets of high density.
It is conceivable that ejection of solar material might have taken place under the influence of certain forces at work within the sun itself, such as electrical repulsion or pressure of light which might become powerful enough under certain conditions to overcome the effect of gravitation.
Next to nothing is known about the physical state of matter at great solar depths, where abnormal conditions of temperature and pressure must exist, and where great physical changes and disturbances may have taken place in the past. Even today solar activity goes through a cycle of change during the sun-spot period, and many millions of years ago the sun-spot cycle of solar activity may have been far different from what it is today and a far more powerful factor in producing changes in the solar system.
Outbursts of novas indicate that agencies making for peace and order are not the only ones at work among the stars. The cause of such outbursts has never been satisfactorily explained. The theory that they are caused by the close approach of two suns or by the encounter of a star with a dark nebula does not account for all of the circumstances of such outbursts. The nebulous matter seen about a nova after the outburst is now generally believed to have been expelled from the star itself at the time of the catastrophe and may conceivably be the stuff of which planetary systems are made.
At some epoch in the past, probably at least one thousand million years ago, our own sun may have undergone some cataclysmic change and this may, conceivably, have been brought about by disturbances within the sun itself. Elements may have been so formed and distributed within the interior of the sun that friction and internal instability resulted and in time produced an upheaval of solar elements with initial velocities so great that, possibly, through electrical repulsion and light-pressure, portions of the ejected streams were permanently detached from the sun and became the nuclei of future planets. In some such way, it is conceivable, our own planet Earth and the other members of our solar system may have been brought into existence in the dim and distant past—many hundred million years ago.