Laplace Nebular Hypothesis

a cosmogonic hypothesis of the formation of the solar system—the sun and the planets and their satellites—from a rotating and contracting gaseous nebula, formulated by P. Laplace in 1796 in his popular book Exposition du système du monde (vols. 1–2).

According to the hypothesis, the diffuse outer part of the nebula (the extended atmosphere of the evolving sun) became increasingly flattened as a result of the speeding up of the rotation during condensation, and when the centrifugal force at the equator became equal to the force of gravity, it acquired a lenticular form. Matter on the sharp edge of the lens no longer continued to condense and remained in place, forming a gaseous disk. It subsequently divided into separate rings, and the matter of each ring collected into a conglomerate that subsequently became a planet. The process just described was frequently repeated during the condensation of these conglomerates, leading to the formation of satellites of the planets. The central conglomerate of the nebula became the sun.

The Laplace nebular hypothesis was unable to explain the slow rotation of the sun, the clockwise rotation of the planets, and the existence of satellites with retrograde motion and of satellites whose period of revolution was less than the period of rotation of the planet. The introduction of modern astrophysical data in the mid-20th century made it possible to develop in new directions Laplace’s idea on the separating out of matter from a condensing protosun as a result of the onset of rotational instability. The mechanism by which the planets formed in this case proved to differ from that proposed by Laplace. The Laplace nebular hypothesis has played a prominent role in the history of science.