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 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.

The Nebular Hypothesis

The nebular hypothesis, developed by Immanuel Kant and given scientific form by P. S. Laplace at the end of the 18th cent., assumed that the solar system in its first state was a nebula, a hot, slowly rotating mass of rarefied matter, which gradually cooled and contracted, the rotation becoming more rapid, in turn giving the nebula a flattened, disklike shape. In time, rings of gaseous matter became separated from the outer part of the disk, until the diminished nebula at the center was surrounded by a series of rings. Out of the material of each ring a great ball was formed, which by shrinking eventually became a planet. The mass at the center of the system condensed to form the sun. The objections to this hypothesis were based on observations of angular momentum that conflicted with the theory.

Contemporary Theories

Contemporary theories return to a form of the nebular hypothesis to explain the transfer of momentum from the central mass to the outer material. The nebula is seen as a dense nucleus, or protosun, surrounded by a thin shell of gaseous matter extending to the edges of the solar system. According to the theory of the protoplanets proposed by Gerard P. Kuiper, the nebula ceased to rotate uniformly and, under the influence of turbulence and tidal action, broke into whirlpools of gas, called protoplanets, within the rotating mass. In time the protoplanets condensed to form the planets. Although Kuiper's theory allows for the distribution of angular momentum, it does not explain adequately the chemical and physical differences of the planets.

Using a chemical approach, H. C. Urey has given evidence that the terrestrial planets were formed at low temperatures, less than 2,200°F (1,200°C). He proposed that the temperatures were high enough to drive off most of the lighter substances, e.g., hydrogen and helium, but low enough to allow for the condensation of heavier substances, e.g., iron and silica, into solid particles, or planetesimals. Eventually, the planetesimals pulled together into protoplanets, the temperature increased, and the metals formed a molten core. At the distances of the Jovian planets the methane, water, and ammonia were frozen, preventing the earthy materials from condensing into small solids and resulting in the different composition of these planets and their great size and low density.

The discovery of extrasolar planetary systems, beginning with 51 Pegasi in 1995, have given planetary scientists pause. Because it was the only one known, all models of planetary systems were based on the characteristics of the solar system—several small planets close to the star, several large planets at greater distances, and nearly circular planetary orbits. However, all of the extrasolar planets are large, many much larger than Jupiter, the largest of the solar planets; many orbit their star at distances less than that of Mercury, the solar planet closest to the sun; and many have highly elliptical orbits. All of this has caused planetary scientists to revisit the contemporary theories of planetary formation.

Below a nice try but rings should be spiral



"There is nothing as impenetrable as a closed mind"
and ..." if everything is a coincidence what is the point of studying or measuring or analyzing anything ?" db



Edited 1 time(s). Last edit at 11/07/2009 08:38PM by Ahatmose.