Laplaces nebular hypothesis

Laplace independently developed a theory about the origin of the solar system that resembled Herschel's sketchy theory of nebular evolution. He first brought out his thesis in 1796, in his Exposition du Système du Monde (contemporaneous English translation: The System of the World), a few years after Herschel's planetary nebula paper appeared. He enlarged on his theory in subsequent editions of his book, and it became widely known as ''Laplace's nebular hypothesis'' or ''nebular theory.''

Laplace's nebular hypothesis played a prominent role in the development of ideas about our solar and Milky Way systems. He envisioned the origin of the solar system as a giant cloud of nebular material, rotating due to some primordial impetus (which he did not explain). As the cloud contracted due to its own gravity, the rotation must, Laplace knew, speed up, just as a twirling ice-skater spins faster as she draws her arms in toward her body. The rotation would gradually flatten the nebular cloud so that it took the shape of a rotating disk.

Laplace claimed that as the disk continued to contract and spin up, rings of gas or nebular material would be left behind, separated from the main disk. Several rings might be spun off successively, in this theory. He further claimed that the material in each ring would gradually draw together to form the planets. The Sun would be left as a remnant of the original spinning cloud, just as, in Herschel's theory, the Sun would form from the inward condensation of an extended nebula.

Laplace's theory to some extent eclipsed Herschel's, although the two men, who met during a visit by Herschel to Paris in 1802, did not consider themselves rivals. Laplace acknowledged their agreement on the basic tenets of the nebular hypothesis in a letter to Herschel in 1814, and incorporated Herschel's views in the 1835 edition of the Exposition du Systeme du Monde. There he noted that Herschel had ''descended'' from a consideration of nebulae in various stages of condensation to the idea that one or more stars might form from an initially amorphous, diffuse nebula; while he, Laplace, had ''ascended'' from a consideration of the probable history of our own solar system to a similar conception of a star-forming nebula.

Variants of Laplace's theory were widely accepted until modern times to account for the structure of the solar system and its planets. As we shall see in subsequent chapters, his theory also shaped astronomers' interpretations of the appearance of some nebulae as flattened systems in apparent rotation.

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