- 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 Laplace in 1796 proposed a similar model, in which a rotating and contracting cloud of gasthe young Sunshed concentric rings of matter that condensed into the planets. 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 matter disk gave rise to the planets through successively larger aglommerations from dust grains through planetesimals and protoplanets, became the prevailing theory of the solar system's origin.
* * *gaseous cloud from which, in the so-called nebular hypothesis of the origin of the solar system, the Sun and planets formed by condensation. In 1755 the German philosopher Immanuel Kant (Kant, Immanuel) suggested that a nebula in slow rotation, gradually pulled together by its own gravitational force and flattened into a spinning disk, gave birth to the Sun and planets. A similar model, but with the planets being formed before the Sun, was proposed by the French astronomer and mathematician Pierre-Simon Laplace (Laplace, Pierre-Simon, marquis de) in 1796. During the late 19th century the Kant-Laplace views were criticized by the British physicist James Clerk Maxwell (Maxwell, James Clerk), who showed that, if all the matter contained in the known planets had once been distributed around the Sun in the form of a disk, the shearing forces of differential rotation would have prevented the condensation of individual planets. Another objection was that the Sun possesses less angular momentum (dependent on the total mass, its distribution, and the speed of rotation) than the theory seemed to require. For several decades most astronomers preferred the so-called collision theory, in which the planets were considered to have been formed as a result of a close approach to the Sun by some other star. Objections to the collision theory more convincing than those against the nebular hypothesis were raised, however, especially as the latter was modified in the 1940s. The mass of the original planet (see protoplanet) was assumed to be larger than in the earlier version of the theory, and the apparent discrepancy in angular momentum was attributed to magnetic forces connecting the Sun and planets. The nebular hypothesis has thus become the prevailing theory of the origin of the solar system.
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