LaPlace's nebular hypothesis is the name given to an ingenious hypothesis proposed by LaPlace, a celebrated French astronomer, in an endeavor to explain how the solar system has been evolved.

You will notice that this is called a hypothesis and not a theory. The word hypothesis is properly applied to a more or less intelligent guess or assumption, that has been made for the purpose of trying to find out in the cause of any natural phenomenon. A theory is an expression of a physical truth based on natural laws and principles that have been independently established. A theory, therefore, is much more complete than a hypothesis. A hypothesis, as Silliman remarks, bears the same relation to a theory or law, that a scaffolding does to a completed building, since it forms a convenient means for erecting the building. LaPlace's work is properly called a hypothesis, because it is not to be considered as any more than a means for enabling one intelligently to inquire into the probable manner in which the solar system has reached its present condition, by gradual steps or stages during the almost inconceivable length of time since its creation.

Before describing LaPlace's hypothesis it will be necessary to give you some ideas concerning what is known by astronomers as the solar system.

The solar system consists of the sun, and the eight large bodies called planets that revolve around the sun. It also includes a number of moons or satellites revolving around the planets, a number of small bodies, called planetoids or asteroids, together with numerous comets and meteorites. Besides these there is probably a system of meteoric bodies that are believed to revolve around the sun, and to produce, by the reflection of the light from their surfaces, what is known as the zodiacal light.

The principal bodies of the solar system are the planets. These constitute eight large bodies named in their order from the sun, beginning with the nearest: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The last four planets, Jupiter, Saturn, Uranus, and Neptune are much larger than the others, and are therefore known as the major planets in order to distinguish them from Mercury, Venus, Earth, and Mars, which are called the minor planets. You can remember the order in which the last three planets come by their initial letter, S-aturn, U-ranus, and N-eptune, spelling the word SUN, around which they all revolve.

It may be interesting to state here that the ancients knew of seven only of these planets. Since, as they asserted, there were only seven days in the week, and seven openings into the head; i. e., two for the eyes, two for the nostrils, two for the ears, and one for the mouth, it was natural that there should be but seven planets. During later years, however, an eighth planet was discovered and named Neptune. It would be interesting to explain to you how the position of this planet was reasoned out by mathematical calculations, that is, in other words, how, as a result of such calculations, an astronomer was told that if he would point his telescope to a certain part of the heavens he would discover a new planet. He did this and located the planet Neptune. However interesting this story may be it belongs properly to astronomy, and will be described in full in the Wonder Book of Astronomy.

In the opinion of some astronomers it is quite probable that a ninth planet will be found far beyond the orbit of Neptune. There may also be some additional planets discovered between Mercury and the Sun.

Besides the eight known planets there exist, somewhere between the orbits of Mars and Jupiter, many smaller planets called asteroids, or minor planets. A long time ago it was pointed out by Bode that a curious relation exists between the distances of the planets from the sun. This relation or law is generally known, after the name of the astronomer who first called attention to it, as Bode's Law. No reason has been discovered for this arrangement of the planets, so that Bode's Law may be regarded as empirical. It may, however, be mentioned here that the distances of all the planets from the sun agrees with the law very closely, with the single exception of Neptune, which is quite at variance with the law.

It was noticed at an early date, that a gap existed between Mars and Jupiter, so that astronomers began to believe that there was probably a missing planet in that space, and this belief was greatly strengthened when Neptune was discovered in 1781. Without going any further into this story in this book, it may be said that it is the general opinion of astronomers that the planetoids or asteroids were formed possibly from the fragments of the missing planet, or, more probably, from the breaking up of some of the outer rings on the planet Mars.

The distances of the planets from the central sun vary from the nearest planet, Mercury, which is about 36,000,000 miles from the sun, to the furthest, or Neptune, which is 2,766,000,000 miles from the sun.

All the major planets have a single moon, or more, revolving around them. For example, Jupiter has four moons; Uranus, six; Saturn, eight; Neptune, one. As to the minor planets, Mars has two moons; and, as far as is known, neither Mercury or Venus has a moon. Our earth has one moon, but, as we shall afterwards see, this is not to be regarded as a moon or satellite of the earth, but rather as a twin planet to the earth.

LaPlace's nebular hypothesis was proposed by LaPlace during the year 1796. While there are many objections that can be brought against it, since it fails to account for all of the phenomena of the solar system, yet it is a significant fact now, in the year 1907, nearly a century and a quarter after the hypothesis was first announced, that although modified in many respects, there has not been any hypothesis proposed to entirely replace it.

While the nebular hypothesis of LaPlace is necessarily a matter that belongs to astronomy, yet it will be advisable to consider it here, since it explains the source of the original heat of both the earth and the moon, which we believe is the true cause of volcanoes.

In his nebular hypothesis, LaPlace assumes that all the materials of which the solar system is formed, were originally scattered throughout space in the shape of an exceedingly rare form of matter known as nebulous matter. He points out that if it be granted that this medium began to accumulate around a common centre, so as to form a huge globe or sphere, and if a motion of rotation on its axis from west to east were given to this sphere that, on strictly mechanical principles, a system of heavenly bodies corresponding to the solar system might have been evolved. Let us, therefore, try to understand how this might have been brought about.

The nebulous matter that LaPlace assumed originally constituted all the matter in the solar system, was highly heated gaseous matter. In other words, it consisted of ordinary matter raised to a very high temperature; LaPlace thought at a temperature very much hotter than that of the sun.

As this great mass of matter commenced to cool, it began to collect around a centre and slowly rotate. Its contraction or shrinkage, while cooling, must have caused an increase in the speed with which it spun around or rotated on its axis. At first it spun but sluggishly, but as it cooled and began to shrink this rate of rotation began slowly to increase.

Now you must bear in mind that the huge rotating mass, as imagined by LaPlace, was very many times larger than the size of our present sun. Indeed, instead of having a diameter of only 866,500 miles, its temperature was so high that the nebulous matter of which it was composed had expanded it so much that it extended far beyond the orbit of Neptune, or had a diameter twice as great as 2,766,000,000 miles.

As the huge mass continued to shrink or contract, its rotation began to gradually increase until at last its centrifugal force was sufficiently great to cause it to bulge out at the equator, so as at last to separate a ring of gaseous matter. This ring was left behind by the sun, as it continued cooling, and formed the first planet that was born into the solar system. The ring might have continued to revolve around the sun for a time, and would, of course, revolve in the same direction as that in which the sun was rotating, that is, from west to east. Eventually, however, it broke up into smaller fragments, that afterwards collected in a single body, and, assuming a globe-like shape of the planet, formed the planet Neptune. Necessarily, too, the planet so formed not only would revolve in its orbit from west to east in the same direction in which the sun was revolving on its axis, but would also rotate or spin on its axis in the same direction.

After, in this way, throwing off the first planet, the central sun continued to cool and grow smaller, until the increase in the rate of its rotation was again such as to permit its centrifugal force to form a second ring around its equator, which being left as the sun continued to contract, gave rise to another planet, or to Uranus, and so on until the four major planets and the four minor planets were born.

According to this hypothesis, the planet that was first born was the planet that is farthest from the sun, that is, Neptune, and the planet last born must have been the nearest planet, Mercury.

But while all this planet forming was going on, the separate planets also continued to shrink, and, therefore, began to rotate more rapidly on their axes. Under the influence of the centrifugal force, ring-like masses began to form around their equators, and these masses left by the planet constituted their moons or satellites. As you can see, according to this hypothesis, just as the planets would all revolve in their orbits from west to east, and rotate on their axes in the same direction as the sun, so, too, the moons or satellites of the planets would also rotate on their axes, from east to west, and revolve in their orbits in the same direction.

In order to show the extent to which LaPlace's nebular hypothesis explains the peculiarities of the solar system, we must inquire what are the most important of these peculiarities. We will take these from Young's general book on Astronomy, from which most of the facts in this chapter have been condensed. They are as follows:

The orbits of nearly all the planets and their satellites are nearly circular; they are all in the same plane; and all revolve in the same direction. They are, moreover, with the single exception of Neptune, arranged at distances from the sun in accordance with Bode's Law.

All the planets increase in both directions, towards and from the sun, in density from Saturn, the least dense.

All the planets, with the exception probably of Uranus, rotate in a plane that is nearly the same as the plane of the orbit in which they revolve. Moreover, with the exception of probably both Uranus and Neptune, all the planets rotate in the same direction as that in which they revolve.

The satellites revolve in orbits whose planes nearly coincide with the plane of the planets' rotation, while the direction of the revolution of the satellites is the same as that in which their planets revolve.

Finally, the largest planets rotate most swiftly.

Now, LaPlace's nebular hypothesis explains nearly all of the above facts. The following modifications of the hypothesis, however, are necessary. Let us briefly examine some of these modifications.

In the first place it can be shown that the original nebulous mass instead of being at a higher temperature than that of the sun was probably at a much lower temperature, since the condensation of the gaseous matter must have increased the temperature. Instead, therefore, of the original nebulous mass being purely gaseous it was, as Young expressed it: "Rather a cloud of ice cold meteoric dust than an incandescent gas or a fire mist." Or in other words, the original nebulous mass from which the solar system was evolved, consisted of finely divided particles of solid or liquid matter surrounded by an envelope of permanent gaseous matter.

A doubt, too, has been raised as regards the manner in which the planets were liberated from the central sun. Instead of separating in the form of a regular ring, it has been thought that probably in most cases this separation assumed the shape of a lump. It might, however, have occurred at times in the ring-like form as may be seen in the case of the planet Saturn.

Again, instead of the outer rings being separated first, and the others in regular order, so that the outer planets are much the older, it would seem possible, or, as Young states, even probable, that several of the planets may be of the same or nearly the same age, as they would be if more than one ring had been separated at one time, or, indeed, several planets may have been formed from different zones of a single ring.

As you will see, LaPlace's nebular hypothesis assumes that both sun and moon were in a highly heated condition when they were separated from the nebulous sun, so that we can understand that the former molten condition of their interiors was due to the heat they originally possessed.