The greatest objection to Laplace's theory is that suns do not seem to break into solar systems, but split in two and form dual suns of more or less equal masses; and the speed of rotation of our Sun has never been sufficient to break it up at all. One of the first evidences of a coming break is a flattening shape such as Jupiter and Saturn have today. There is not a sign of it on the Sun. Small streams of gases do not form nuclei, while large streams do; and mathematically Laplace's hypothesis does not justify the formation of planets, though it does account for the suns.
To overcome these objections, T. C. Chamberlin and F. R. Moulton developed in 1900 a more probable solution which has been elaborated by Sir James Jeans and others under the title of "the tidal theory." They agreed with Laplace for the formation of nebulae, but from there on they differ and assume that our Sun came within the sphere of influence of another star or nebula as they both moved through space. Just as the Moon raises tides on both sides of the Earth, so this much larger visitor raised enormous tides on the Sun. Long arms of gaseous matter protruded until the formation began to look like the large double pinwheel in Canes Venatici.
Nuclei formed in the arms. As the visitor left the district his influence ceased but the damage he had caused remained. The protruding gases began to cool and little planets were formed. The smallest planets cooled first, the largest last. In addition there were vast numbers of tiny particles whirling around in their own highly eccentric orbits. These little scraps of matter often collided with the baby planets and added their mass. So frequent were these collisions and so great were the contributions made in this way, that the larger part of any planet is thought to be made up of these gathered in waifs. In their honor Chamberlin and Moulton call their whole hypothesis "the planetesimal theory."
Not all the tiny particles (or planetesimals) found homes in this convenient fashion. Many gathered together in the less permanent residence of comets and others are still on the loose. But there were in the beginning of our planetary systems far more than there are today. The motion of the young planets was retarded by passing through the dust, as well as by their own rapidly increasing size, so that while they started off in rather elongated ellipses, they tend to move in more and more circular fashion.
While the planets were forming, tides were caused on them by their nearest neighbors, and arms were sent out just as before. These in turn broke up, and formed a number of satellites, or condensed without breaking up and formed just one, like our Moon.
The same theory must apply throughout the universe. The solar system itself is rather complicated, so it is hardly fair to expect any theory that covers its formation to be simple. All the general rules fall down in particular cases, and the variation among the individual stars is enormous. Some, such as Antares, are so tenuous that we believe them to be composed of gases under very low pressure, while others are much denser than any materials, with which we are actually acquainted. The densest substance known on Earth is iridium, which has a specific gravity of 22.4 compared to water as 1, but that is not the reason why it is alloyed with platinum to make our fountain pen points. The faint companion of Sirius has a specific gravity of about 60,000. The only explanation of such tremendous density is that the particles of matter have been squeezed together, eliminating intervening space, in a way that has never been approached on Earth.
The extremely high temperatures noticed on some of the heavy stars, and on others too, may be the cause of this density. Star temperatures rise into millions of degrees Centigrade, where a few thousand only can be reached on Earth. In such heat, matter could exist only in its simplest forms, and it may be compacted beyond anything we know.
The solar system, in spite of its insularity and lack of connection with the outside world, is in reality part of a much greater archipelago--the Milky Way, more scientifically known as the "Galaxy." The whole group is arranged somewhat like a lens. Anyone standing inside a lens would see more glass along the long diameter than he would when looking across it. Thus the long axis gives the effect of the Milky Way from our Earth. This great circle through the heavens is composed of a multitude of stars revolving like a wheel 250,000 light years in diameter, or more properly like the horses on a race track, with the inner ones always traveling faster and arriving first back at the starting point, just as the inner planets can gallop faster than the outer ones around the Sun. The Sun (the Earth is far too small to be considered separately in this reckoning) is 40,000 light years from the center. In 230,000,000 years our solar portion completes a single circuit; but the outer ranges of this vast wheel take 530,000,000 years to revolve once around its focal point.
Outside the Galaxy are found other giant nebulae, some of them with diameters as great as our own Milky Way's. They are all more than 800,000 light years distant from us. They may be considered therefore as fairly separate and distinct systems, each one probably containing suns, planets, satellites and all the other stray bits of heavenly matter in the making.
The Copernican theory has opened limitless possibilities of space and time. We know today that we are not the center of a hollow sphere. The thought of worlds beyond worlds is more than the mind can grasp. Since the time of Copernicus and since the time of Shakespeare the universe has opened both inwardly and outwardly before our eyes; but if there is a limit to our knowledge we have not yet glimpsed its margin. In our own lifetime far stranger thoughts than any of these have been enunciated, and as strangers we must bid them welcome. There are still more things in heaven and Earth than, we, twentieth-century Horatios, have dreamt in all the far realms of our philosophy.