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Authors: Isaac Asimov
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point,all the planets known to Laplace moved about the Sun in a counterclockwise direction, and all the planets whose rotations were known rotated about their axes in a counterclockwise direction. Added to that was the fact that all the satellites known to Laplace revolved about their planets in a counterclockwise direction.
Finally, all the planets had orbits that were nearly in the plane of the Sun’s equator and all the satellites had orbits that were nearly in the plane of their planet’s equator.
To account for all this, Laplace suggested that the Solar system was originally a vast cloud of dust and gas called a nebula (from the Latin word for
cloud
). The nebula was turning slowly in a counterclockwise direction. Its own gravitational field slowly contracted it, and as it contracted it had to spin faster and faster in accordance with something called the law of conservation of angular momentum. Eventually, it condensed to form the Sun, which is still spinning in the counterclockwise direction.
As the cloud contracted on its way to the Sun and as it increased its rate of spin, the centrifugal effect of rotation caused it to belly out at its equator. (This happens to the Earth, which has an equatorial bulge that lifts points on its equator 13 miles farther from the center of the Earth than the north and south poles are.)
The bulge of the contracting nebula became more and more pronounced as it shrank further and speeded up further, until the entire bulge was thrown off like a thin doughnut around the contracting nebula. As the nebula continued to shrink, additional doughnuts of matter were shed.
Each doughnut, in Laplace’s view, gradually condensed into a planet, maintaining the original counterclockwise spin, and speeding up that spin as it condensed. As each planet formed there was a chance it might shed smaller subsidiary doughnuts of its own, which became the satellites. The rings around Saturn are examples of matter that has been given off (according to Laplace’s nebular hypothesis) and has not yet condensed to a satellite.
The nebular hypothesis explains why all the revolutions and rotations in the Solar system should be in the same direction. * It is because all participate in the spin of the original nebula.
It also explains why all the planets revolve in the plane of the Sun’s equator. It is because it is from the Sun’s equatorial regions that they were originally formed; and it is from the planetary equatorial regions that the satellites formed.
The nebular hypothesis was more or less accepted by astronomers during the nineteenth century, and it added detail to the picture that people drew of Mars and Venus.
As the nebula condensed, according to this theory, it would seem that the planets would form in order from the outermost to the innermost. In other words, after the nebula had condensed to the point where it was only 500,000,000 kilometers (310,000,000 miles) across, it gave off the ring of matter that formed Mars. Then, after considerable time taken up in further contraction, it gave off the matter that formed the Earth and the Moon, and after another unknown length of time, the matter that formed Venus.
By the nebular hypothesis, therefore, it would seem that Mars was considerably older than Earth, and that Earth was considerably older than Venus.
It became customary, therefore, to think of Mars as having moved farther along the evolutionary path than Earth; not only with respect to its planetary characteristics, but with respect to the life upon it. Similarly, Venus had not moved as far along the evolutionary path. Thus, the Swedish chemist Svante August Arrhenius (1859–1927) drew an eloquent picture in 1918 of Venus as a water-soaked jungle.
This sort of thinking was reflected in science fiction stories, which very often depicted Mars as occupied by an intelligent race with a long history that dwarfed that of Earthly human beings. The Martians were pictured as far advanced beyond
Finally, all the planets had orbits that were nearly in the plane of the Sun’s equator and all the satellites had orbits that were nearly in the plane of their planet’s equator.
To account for all this, Laplace suggested that the Solar system was originally a vast cloud of dust and gas called a nebula (from the Latin word for
cloud
). The nebula was turning slowly in a counterclockwise direction. Its own gravitational field slowly contracted it, and as it contracted it had to spin faster and faster in accordance with something called the law of conservation of angular momentum. Eventually, it condensed to form the Sun, which is still spinning in the counterclockwise direction.
As the cloud contracted on its way to the Sun and as it increased its rate of spin, the centrifugal effect of rotation caused it to belly out at its equator. (This happens to the Earth, which has an equatorial bulge that lifts points on its equator 13 miles farther from the center of the Earth than the north and south poles are.)
The bulge of the contracting nebula became more and more pronounced as it shrank further and speeded up further, until the entire bulge was thrown off like a thin doughnut around the contracting nebula. As the nebula continued to shrink, additional doughnuts of matter were shed.
Each doughnut, in Laplace’s view, gradually condensed into a planet, maintaining the original counterclockwise spin, and speeding up that spin as it condensed. As each planet formed there was a chance it might shed smaller subsidiary doughnuts of its own, which became the satellites. The rings around Saturn are examples of matter that has been given off (according to Laplace’s nebular hypothesis) and has not yet condensed to a satellite.
The nebular hypothesis explains why all the revolutions and rotations in the Solar system should be in the same direction. * It is because all participate in the spin of the original nebula.
It also explains why all the planets revolve in the plane of the Sun’s equator. It is because it is from the Sun’s equatorial regions that they were originally formed; and it is from the planetary equatorial regions that the satellites formed.
The nebular hypothesis was more or less accepted by astronomers during the nineteenth century, and it added detail to the picture that people drew of Mars and Venus.
As the nebula condensed, according to this theory, it would seem that the planets would form in order from the outermost to the innermost. In other words, after the nebula had condensed to the point where it was only 500,000,000 kilometers (310,000,000 miles) across, it gave off the ring of matter that formed Mars. Then, after considerable time taken up in further contraction, it gave off the matter that formed the Earth and the Moon, and after another unknown length of time, the matter that formed Venus.
By the nebular hypothesis, therefore, it would seem that Mars was considerably older than Earth, and that Earth was considerably older than Venus.
It became customary, therefore, to think of Mars as having moved farther along the evolutionary path than Earth; not only with respect to its planetary characteristics, but with respect to the life upon it. Similarly, Venus had not moved as far along the evolutionary path. Thus, the Swedish chemist Svante August Arrhenius (1859–1927) drew an eloquent picture in 1918 of Venus as a water-soaked jungle.
This sort of thinking was reflected in science fiction stories, which very often depicted Mars as occupied by an intelligent race with a long history that dwarfed that of Earthly human beings. The Martians were pictured as far advanced beyond
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