solar system: The Planetesimal and Tidal Theories

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. C. Chamberlin and F. R. Moulton in the early part of the 20th cent., a star passed close to the sun. 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 1918, 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.

Sections in this article:

• Introduction
• Contemporary Theories
• The Planetesimal and Tidal Theories
• The Nebular Hypothesis
• Origin of the Solar System
• Physical Properties
• Planetary Motion
• The Planets
• Bibliography

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