The Nebular Hypothesis - Of the many attempts to explain the origin of the solar system none has met with more favor than that known as the nebular hypothesis, which assumes that at one time all the material in the solar system existed in the form of a rotating mass of nebulous gas that occupied all the space from the center of the present sun out to and beyond the limits of the orbit of the outermost planet Neptune.
Under the universal law of gravitation, by which every particle of matter in the universe attracts every other particle, these gas particles were attracted towards a common center. In the course of time a portion of the mass was separated in the form of a ring, or, as some say, as an irregular mass, which in time, by its rotation on its own axis, formed a spheroidal body revolving around the central mass.
This was the planet Neptune, which continued to revolve around the central mass from which in turn the other planets and their satellites were separated, the earth being the sixth one and Mercury the last one. The residual central mass is the sun, which, according to the hypothesis, is still contracting. The planetary masses probably separated from the parent mass while still in the gaseous condition, but continued to contract until they became liquid and on further cooling, solid, at least on the surface.
This hypothesis, with sundry modifications, has been widely accepted because it seemed to account for so many things about the system. Recently many objections to this explanation have been raised, while another explanation has been growing in favor with some people because it appears to be free from some of the difficulties in the nebular hypothesis.
The Planetesimal Hypothesis - The planetesimal hypothesis, although it starts with a nebulous mass, differs radically from the nebular hypothesis in a number of particulars. Unlike the first, however, the nebula is not necessarily a gas nor is it highly heated and hence it need not pass through a liquid state. The hypothesis starts with a spiral nebula, which is one of the most common forms in the sky at present.
The knots or denser portions in the nebula are the nuclei of the future planets and satellites, and the nebulous haze surrounding the nuclei consists of finely divided matter mostly solid, possibly some liquid and gaseous, which is later gathered in by gravitation, and added to the nuclei to form the planets. All of the material, first in the nebula and later in the planets and satellites, moves about the central mass in elliptical orbits. It all has a double motion, first around the central axis of its own nucleus or planet, and second around the central sun.
The hypothesis supposes a relatively slow growth of the earth, as of the other planets, with increasing temperature in the central portions due to gravity. That is, a large body will have greater pressure by gravity at the center than a small one and, hence, will have greater heat induced by the pressure. In a body as large as the earth, the gravitative attraction of all the particles towards the center produces an enormous pressure on the central portions, a pressure sufficient to produce heat and raise the temperature of the interior.
For a long time after the earth nuclues began to grow it was too small to have an atmosphere or even a hydrosphere, both of which formed gradually as soon as the planet was large enough to hold them by the force of gravity. They would be increased by the parts expelled from the interior by gravity pressure, as well as the parts that would be drawn to the surface of the mass from the surrounding nebula. Hence, the accretion of the planetesimal matter of the outer half or more of the earth would be through an atmosphere and subject to the action of moisture.
This hypothesis likewise makes possible a much longer period of time in which life was possible on the earth or in which the conditions favored the existence of life in the initial stages, than does the nebular hypothesis. The principal points of difference between the two hypotheses are, that according to the first, the earth passes from a highly heated gaseous condition through a hot molten state to the present solid condition, while according to the second, the earth was never entirely gaseous, never necessarily molten and possibly never much hotter than at present.
By the first, the earth was once larger than at present and included the moon which was later separated from it; by the second, the earth was never larger, probably not so large in the past as at the present. By the first, the outer planet, Neptune, is the oldest and the inner one. Mercury the youngest; by the second, the planets and their satellites are all of about the same age, that is, they were all in process of formation at the same time.
According to the planetesimal hypothesis the moon is devoid of water and an atmosphere because it is too small to hold them on the surface by gravity, and not because it is so old that it has lost them as sometimes claimed in the nebular hypothesis.
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