Kant's central idea was that the solar system began as a cloud of dispersed particles. He assumed that the mutual gravitational attractions of the particles caused them to start moving and colliding, at which point chemical forces kept them bonded together. As some of these aggregates became larger than others, they grew still more rapidly, ultimately forming the planets. Because Kant was highly versed in neither physics nor mathematics, he did not recognize the intrinsic limitations of his approach. His model does not account for planets moving around the Sun in the same direction and in the same plane, as they are observed to do, nor does it explain the revolution of planetary satellites.
A significant step forward was made by Pierre-Simon Laplace of France some 40 years later. A brilliant mathematician, Laplace was particularly successful in the field of celestial mechanics. Besides publishing a monumental treatise on the subject, he wrote a popular book on astronomy, with an appendix in which he made some suggestions about the origin of the solar system.
Laplace's model begins with the Sun already formed and rotating and its atmosphere extending beyond the distance at which the farthest planet would be created. Knowing nothing about the true source of energy in stars, Laplace assumed that the Sun would start to cool as it radiated away its heat. In response to this cooling, as the pressure exerted by its gases declined, the Sun would contract. According to the law of conservation of angular momentum, the decrease in size would be accompanied by an increase in the Sun's rotational velocity. Centrifugal acceleration would push the material in the atmosphere outward, while gravitational attraction would pull it toward the central mass. When these forces just balanced, a ring of material would be left behind in the plane of the Sun's equator. This process would have continued through the formation of several concentric rings, each of which then would have coalesced to form a planet. Similarly, a planet's moons would have originated from rings produced by the forming planets.
Laplace's model led naturally to the observed result of planets revolving around the Sun in the same plane and in the same direction as the Sun rotates. Because the theory of Laplace incorporated Kant's idea of planets coalescing from dispersed material, their two approaches are often combined in a single model called the Kant-Laplace nebular hypothesis. This model for solar system formation was widely accepted for about
100 years. During this period, the apparent regularity of motions in the solar system was contradicted by the discovery of asteroids with highly eccentric orbits and moons with retrograde orbits. Another problem with the nebular hypothesis was the fact that, whereas the Sun contains 99.9 percent of the mass of the solar system, the planets (principally the four giant outer planets) carry more than 99 percent of the system's angular momentum. For the solar system to conform to this theory, either the Sun should be rotating more rapidly or the planets should be revolving around it more slowly.
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