Nebular Hypothesis

Nebular Hypothesis, an explanation of how the solar system was formed, proposed by Pierre Simon de Laplace in 1796. Laplace said that the material from which the solar system was formed was once a slowly rotating cloud, or nebula, of extremely hot gas. The gas cooled and the nebula began to shrink. As the nebula became smaller, it rotated more rapidly, becoming somewhat flattened at the poles.

A combination of centrifugal force, produced by the nebula's rotation, and gravitational force, from the mass of the nebula, caused rings of gas to be left behind as the nebula shrank. These rings condensed into planets and their satellites, while the remaining part of the nebula formed the sun.

The nebular hypothesis, widely accepted for about a hundred years, has several serious flaws. The most serious concerns the speed of rotation of the sun. When the nebular hypothesis is worked out mathematically on the basis of the known orbital momentum of the planets, it predicts that the sun must rotate about 50 times more rapidly than it actually does. There is also some doubt that the rings pictured by Laplace would ever condense into planets.

In the early 20th century, the nebular hypothesis was rejected and the planetesimal hypothesis, that the planets were formed from material drawn out of the sun, became popular. This theory, too, proved unsatisfactory. Later theories have revived the concept of a nebular origin for the planets, but not in the same form in which it was proposed by Laplace.