nebular hypothesis(redirected from Near-collision theory)
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nebular hypothesis:see solar systemsolar system,
the sun and the surrounding planets, natural satellites, dwarf planets, asteroids, meteoroids, and comets that are bound by its gravity. The sun is by far the most massive part of the solar system, containing almost 99.9% of the system's total mass.
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nebular hypothesis(neb -yŭ-ler) A theory for the origin of the Solar System put forward by the French mathematician Pierre-Simon de Laplace in 1796 and similar to a suggestion of the philosopher Immanuel Kant in 1755. It was proposed that the Solar System formed from a great rotating cloud, or nebula, of material (gas and dust) collapsing under its own gravitational attraction. An interaction of forces caused the cloud to form a rotating flattened disk, called the solar nebula. To conserve angular momentum, the disk rotated more rapidly as the contraction progressed. Laplace suggested that rings of material became detached from the spinning disk when the velocity at its edge exceeded a critical value, and that the material in these rings later coalesced to form the planets. The central product of the contraction is the Sun, while the planetary satellites may have formed from further rings shed by the condensing planets.
The hypothesis, popular throughout the 19th century, went out of favor. The main problem was that it indicated that the Sun should still be spinning on the verge of rotational instability; it could not explain why the Sun has almost 99.9% of the mass of the Solar System but only about 2% of the total angular momentum. In addition, calculations showed that the rings would not condense to form planets. However, in a modified form, it is the basis of most modern ideas for the formation of the Sun and planets. See Solar System, origin. Compare encounter theories.
a cosmogonical hypothesis which assumes that the solar system (and celestial bodies in general) was formed out of a rarefied nebula. The term “nebular hypothesis” originated in the 19th century in connection with the Laplace nebular hypothesis. Later, the term was also used in Kant’s hypothesis and in other theories that assumed the formation of celestial bodies from nebulae of gas or dust. The term “nebular hypothesis” is not usually used in relation to modern cosmogonical hypotheses. (SeeCOSMOGONY.)