solar nebula

Gaseous cloud from which, in the nebular hypothesis of the origin of the solar system, the Sun and planets formed by condensation. In 1755 Immanuel Kant suggested that a nebula gradually pulled together by its own gravity developed into the Sun and planets. Pierre-Simon, marquis de Laplace, in 1796 proposed a similar model, in which a rotating and contracting cloud of gas—the young Sun—shed concentric rings of matter that condensed into the planets. But James Clerk Maxwell showed that, if all the matter in the known planets had once been distributed this way, shearing forces would have prevented such condensation. Another objection was that the Sun has less angular momentum than the theory seems to require. In the early 20th century most astronomers preferred the collision theory: that the planets formed as a result of a close approach to the Sun by another star. Eventually, however, stronger objections were mounted to the collision theory than to the nebular hypothesis, and a modified version of the latter—in which a rotating disk of matter gave rise to the planets through successively larger agglomerations, from dust grains through planetesimals and protoplanets—became the prevailing theory of the solar system's origin.

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