nebular hypothesis
(redirected from Near-collision theory)Also found in: Dictionary, Thesaurus.
solar system
solar 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. The principal members of the sun's retinue are the eight major planets; other parts of the solar system are discussed in separate articles: see asteroid,comet, dwarf planet, and meteor.
The Planets
In order of increasing average distance from the sun, the planets are Mercury, Venus, earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The planets orbiting nearer the sun than the earth are termed inferior planets; those whose orbits are larger are called superior planets. The unit for measuring distance in the solar system is the astronomical unit (AU), the average distance between the earth and the sun. The mean distances of the planets from the sun range from 0.39 AU for Mercury to 30.04 AU for Nepture.
Pluto, regarded for many years after its discovery as a planet, was reclassified in 2006 as a dwarf planet, which is a planetlike celestial body that does not clear or dominate the region of its orbit. In addition, Pluto is unlike the terrestrial planets—Mercury, Venus, Earth, and Mars—which are rocky, and it is unlike the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune). Its orbit, which is tilted from the plane in which the eight planets travel about the Sun, its size, and its composition more closely resemble those of the objects residing in the Kuiper belt (which were first discovered in 1992; see under comet) than those of a major planet, and Pluto is now recognized as a Kuiper belt, or transneptunian, object.
See the table entitled Major Planets of the Solar System.
Planetary Motion
Physical Properties
Origin of the Solar System
The Nebular Hypothesis
The Planetesimal and Tidal Theories
Encounter or collision theories, in which a star passes close by or actually collides with the sun, try to explain the distribution of angular momentum. According to the planetesimal theory developed by T. C. Chamberlin and F. R. Moulton in the early part of the 20th cent., a star passed close to the sun. Huge tides were raised on the surface; some of this erupted matter was torn free and, by a cross-pull from the star, was thrust into elliptical orbits around the sun. The smaller masses quickly cooled to become solid bodies, called planetesimals. As their orbits crossed, the larger bodies grew by absorbing the planetesimals, thus becoming planets.
The tidal theory, proposed by James Jeans and Harold Jeffreys in 1918, is a variation of the planetesimal concept: it suggests that a huge tidal wave, raised on the sun by a passing star, was drawn into a long filament and became detached from the principal mass. As the stream of gaseous material condensed, it separated into masses of various sizes, which, by further condensation, took the form of the planets. Serious objections against the encounter theories remain; the angular momentum problem is not fully explained.
Contemporary Theories
Contemporary theories return to a form of the nebular hypothesis to explain the transfer of momentum from the central mass to the outer material. The nebula is seen as a dense nucleus, or protosun, surrounded by a thin shell of gaseous matter extending to the edges of the solar system. According to the theory of the protoplanets proposed by Gerard P. Kuiper, the nebula ceased to rotate uniformly and, under the influence of turbulence and tidal action, broke into whirlpools of gas, called protoplanets, within the rotating mass. In time the protoplanets condensed to form the planets. Although Kuiper's theory allows for the distribution of angular momentum, it does not explain adequately the chemical and physical differences of the planets.
Using a chemical approach, H. C. Urey has given evidence that the terrestrial planets were formed at low temperatures, less than 2,200℉ (1,200℃). He proposed that the temperatures were high enough to drive off most of the lighter substances, e.g., hydrogen and helium, but low enough to allow for the condensation of heavier substances, e.g., iron and silica, into solid particles, or planetesimals. Eventually, the planetesimals pulled together into protoplanets, the temperature increased, and the metals formed a molten core. At the distances of the Jovian planets the methane, water, and ammonia were frozen, preventing the earthy materials from condensing into small solids and resulting in the different composition of these planets and their great size and low density.
The discovery of extrasolar planetary systems, beginning with 51 Pegasi in 1995 and now numbering in the hundreds, have given planetary scientists pause. Because it was the only one known, all models of planetary systems were based on the characteristics of the solar system—several small planets close to the star, several large planets at greater distances, and nearly circular planetary orbits. However, all of the extrasolar planets are large, many much larger than Jupiter, the largest of the solar planets; many orbit their star at distances less than that of Mercury, the solar planet closest to the sun; and many have highly elliptical orbits. All of this has caused planetary scientists to revisit the contemporary theories of planetary formation.
Bibliography
See N. Booth, Exploring the Solar System (1996); P. R. Weissman et al., ed., Encyclopedia of the Solar System (1998); J. K. Beatty et al., ed., The New Solar System (4th ed. 1999); B. W. Jones, Discovering the Solar System (1999).
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.
Nebular Hypothesis
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.)