The origin of the solar system
The first attempt to apply dynamical principles to the problem of the origin of our planetary system was by Edouard Roche, in 1854. His procedure was to make more specific the Nebular Hypothesis, which had been stated in a very indefinite qualitative form in a popular work by Laplace. He began by considering the sun with its present mass and with a radius already much less than that of the orbit of Mercury. It contained much the greater part of the mass of the system, and produced a gravitational field that dominated the rest of the system, as at present. It was surrounded by a rarefied nebula, which rotated like a rigid body: that is, every part of it took the same time to revolve about the central mass. As the system cooled, the gaseous envelope contracted, and the more it contracted the faster it rotated. This is in accordance with the same dynamical law as accounts for the rapid swirl developed by the water in a bath near the outlet, or for that in a cyclone or tornado; when the matter in a system approaches the centre, it gains in speed of rotation. But the faster it rotated the greater became the force required to prevent a particle at the outside from becoming detached from the main mass. The reduction in size also made gravity at the outside greater, but it can be shown that the force required to hold the mass together would increase faster than gravity would, and a stage would be reached when a ring of matter would be detached around the equator. As time went on several rings were detached, and each ring ultimately condensed to form a planet. The planets themselves developed in a similar way, and most of the larger ones produced families of satellites by a similar process on a smaller scale.
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