Mankind's Version: Planetary Formation

Mankind’s Version: Planetary Formation

“Nebular Hypothesis. One of the first modern speculations on the origin of the solar system was that of German philosopher Immanuel Kant, who in 1755 proposed its creation from a spinning gaseous nebula. He suggested that the center of the whirling gas cloud formed the sun and that the outer parts formed the planets. Kant had observed spiral galaxies through telescopes and taken them to be such spinning nebulae. The French astronomer and mathematician Pierre Laplace developed a similar theory, in which he suggested that all planets originally formed from hot gases. The Kant-Laplace nebular hypothesis was generally accepted by astronomers until the end of the 19th century.

Chamberlin-Moulton Theory. About 1900 the U. S. geologist T. C. Chamberlin, together with the astronomer F. R. Moulton, worked out another theory. They proposed that the solar system was formed when another star passed close to the sun- an event that was recognized as very unlikely. The near encounter was supposed to have out solar material by tidal action, and this hot gas condensed into planetesmials such as the asteroids observed today. The planetesimals were then supposed to have drawn together by mutual gravitational attraction to form the planets.

Other men, such as the English scientist James Jeans, attempted to improve on the near-collision theory. However, it was soon shown that the material supposedly pulled from the sun by the passing star would not have enough angular momentum to remain in orbit as planets and asteroids. In 1934 the U. S. astrophysicist H. N. Russell pointed out that the planets have less than 0.1% of the mass of the solar system and yet have 98% of the angular momentum. Astronomers meanwhile were finding evidence that all stars form from the contraction of clouds of interstellar dust and gas. Since the sun almost certainly formed this way as well, theorists returned to the earlier nebular hypothesis of Kant and Laplace.

Dust-Cloud Theory. Between 1940 and 1955 the German astronomer Carl f. von Weizsaccker, the Dutch-American astronomer Gerald P. Kuiper and the U. S. chemist Harold C. Urey worked out a theory that attempted to account for all the characteristics of the solar system that need to be explained. According to their dust-cloud theory, the solar system was formed from a slowly rotating cloud of dust and gas that contracted and started to rotate faster in its outer parts, where eddies formed. These eddies were small near the center of the cloud and larger at greater distances from the center. The distances corresponded more or less to the Titius-Bode relation.

As the clouds cooled, materials coagulated near the edges of the eddies and eventually formed planets and asteroids, all moving in the same direction. The slowly rotating central part of the cloud condensed and formed the sun, and the sun’s central temperature rose as gravity further compressed the material. When nuclear reactions eventually began in the suns interior, about 5 billion years ago, much of the nearby gas was blown away by the pressure of the sun’s emitted light. Nevertheless the earthy retained an atmosphere consisting of methane, ammonia, carbon monoxide, water vapor, and nitrogen, with perhaps some hydrogen. In this primitive atmosphere and in the seas below it, organic compounds were formed that eventually resulted in living organisms. The organisms evolved in the next 2 billion years into higher plants and animals, and photosynthesis by plants and the weathering of rock produced the oxygen in the earth’s atmosphere.

Although free gases near the sun were blown outward 4 to 5 billion years ago, according to the dust-cloud theory, the giant planets were too distant to be much affected. They are large, therefore, and contain a great amount of hydrogen. The comets, in turn, are thought to be the outer part of the primordial nebula, left behind as the inner part condensed to form the sun and the planets. The Dutch astronomer J. H. Oort speculated that this material condensed into chunks that continue to move along with the sun through space. Now and then a chunk is perturbed and falls slowly toward the sun. As it is heated by sunlight, it grows a coma and tail.

The dust-cloud theory thus explains the solar system characteristics listed above. It is most weak in detailing the process whereby the planets and asteroids formed from solids that made up only a small percent of the primordial nebula. However, this is essentially a chemical problem, strongly dependent on the sequence or timing of events such as eddy formation, temperature changes, and the start of solar luminosity.”

THORNTON PAGE, Co-author of “Neighbors of the Earth.”

Bibliography

Beatty, J. Kelly, and others, eds., The New Solar System, 2d ed. (Cambridge 1982).

Hardy, David A., Atlas of the Solar System (David and Charles 1983).

Hartmann, William K., Moons and Planets, 2d. ed. (Wadsworth 1983).

Jones, B. W., and Keynes, Milton, The Solar System, 4 vols. (Pergamon 1984).

Whipple, Fred L., Orbiting the Sun: Planets and Satellites of the Solar System (Harvard Univ. Press 1981).

Return to Main Document

Return to Main Menu