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
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.
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
THORNTON PAGE, Co-author of
“Neighbors of the Earth.”
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).
Information acquired within the quotes is from: Encyclopedia Americana- International Edition copyright 1994 by Grolier Incorporated, Volume 25 pages 196-197
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