Planetesimal And Tidal Theory Pdf


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planetesimal and tidal theory pdf

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These objects coagulate by collisions, eventually building planetary cores.

M Woolfson discusses theories of how the Sun and the planets began.

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Planetesimals

Besides explaining the birth of the sun, planets, dwarf planets, moons, asteroids, and comets, a theory of the origin of the solar system must explain the chemical and physical differences of the planets; their orbital regularities, i.

The nebular hypothesis, developed by Immanuel Kant and given scientific form by P. Laplace at the end of the 18th cent. 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. Encounter or collision theories, in which a star passes close by or actually collides with the sun, try to explain the distribution of angular momentum.

According to the planetesimal theory developed by T. Chamberlin and F. Moulton in the early part of the 20th cent. Huge tides were raised on the surface; some of this erupted matter was torn free and, by a cross-pull from the star, was thrust into elliptical orbits around the sun. The smaller masses quickly cooled to become solid bodies, called planetesimals. As their orbits crossed, the larger bodies grew by absorbing the planetesimals, thus becoming planets.

The tidal theory, proposed by James Jeans and Harold Jeffreys in , is a variation of the planetesimal concept: it suggests that a huge tidal wave, raised on the sun by a passing star, was drawn into a long filament and became detached from the principal mass. As the stream of gaseous material condensed, it separated into masses of various sizes, which, by further condensation, took the form of the planets.

Serious objections against the encounter theories remain; the angular momentum problem is not fully explained. 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. Urey has given evidence that the terrestrial planets were formed at low temperatures, less than 2,C 1,C.

He proposed that the temperatures were high enough to drive off most of the lighter substances, e. 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 and now numbering in the hundreds, 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?

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.

The Columbia Electronic Encyclopedia, 6th ed. Copyright , Columbia University Press. All rights reserved. See more Encyclopedia articles on: Astronomy: General. Enter your search terms:.

Earth, Moon and Planets

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Ergebnisse der Exakten Naturwissenschaften pp Cite as. The first attempt to apply dynamical principles to the problem of the origin of our planetary system was by Edouard Roche , in 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.

The origin of the solar system

Besides explaining the birth of the sun, planets, dwarf planets, moons, asteroids, and comets, a theory of the origin of the solar system must explain the chemical and physical differences of the planets; their orbital regularities, i. The nebular hypothesis, developed by Immanuel Kant and given scientific form by P. Laplace at the end of the 18th cent. 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.

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