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The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System. It suggests that the Solar System formed from nebulous material. The theory was developed by Immanuel Kant and published in his Universal Natural History and Theory of the Heaven. Originally applied to our own Solar System, this process of planetary system formation is now thought to be at work throughout the Universe.〔 The widely accepted modern variant of the nebular hypothesis is the solar nebular disk model (SNDM) or simply solar nebular model.〔 This nebular hypothesis offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the nebular hypothesis are echoed in modern theories of planetary formation, but most elements have been superseded. According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). These clouds are gravitationally unstable, and matter coalesces within them to smaller denser clumps, which then rotate, collapse, and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A Sun-like star usually takes approximately 1 million years to form, with the protoplanetary disk evolving into a planetary system over the next 10-100 million years.〔 The protoplanetary disk is an accretion disk that feeds the central star. Initially very hot, the disk later cools in what is known as the T tauri star stage; here, formation of small dust grains made of rocks and ice is possible. The grains eventually may coagulate into kilometer-sized planetesimals. If the disk is massive enough, the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes approximately 100 million to a billion years.〔 The formation of giant planets is a more complicated process. It is thought to occur beyond the so-called frost line, where planetary embryos mainly are made of various types of ice. As a result, they are several times more massive than in the inner part of the protoplanetary disk. What follows after the embryo formation is not completely clear. Some embryos appear to continue to grow and eventually reach 5–10 Earth masses—the threshold value, which is necessary to begin accretion of the hydrogen–helium gas from the disk. The accumulation of gas by the core is initially a slow process, which continues for several million years, but after the forming protoplanet reaches about 30 Earth masses () it accelerates and proceeds in a runaway manner. Jupiter- and Saturn-like planets are thought to accumulate the bulk of their mass during only 10,000 years. The accretion stops when the gas is exhausted. The formed planets can migrate over long distances during or after their formation. Ice giants such as Uranus and Neptune are thought to be failed cores, which formed too late when the disk had almost disappeared.〔 == History == (詳細はEmanuel Swedenborg.〔http://www.newchurchhistory.org/articles/glb2007/baker.pdf〕 Immanuel Kant, who was familiar with Swedenborg's work, developed the theory further in 1755, when Kant published his ''Universal Natural History and Theory of the Heavens'', wherein he argued that gaseous clouds, nebulae, slowly rotate, gradually collapse and flatten due to gravity, eventually forming stars and planets.〔 For details of Kant's position, see Stephen Palmquist, "Kant's Cosmogony Re-Evaluated", ''Studies in History and Philosophy of Science'' 18:3 (September 1987), pp.255-269.〕 A similar model was developed independently and proposed in 1796 by Pierre-Simon Laplace.〔 in his ''Exposition du systeme du monde''. He envisioned that the Sun originally had an extended hot atmosphere throughout the volume of the Solar System. His theory featured a contracting and cooling protosolar cloud—the protosolar nebula. As this cooled and contracted, it flattened and spun more rapidly, throwing off (or shedding) a series of gaseous rings of material; and according to him, the planets condensed from this material. His model was similar to Kant's, except more detailed and on a smaller scale.〔 While the Laplacian nebular model dominated in the 19th century, it encountered a number of difficulties. The main problem was angular momentum distribution between the Sun and planets. The planets have 99% of the angular momentum, and this fact could not be explained by the nebular model.〔 As a result, this theory of planet formation was largely abandoned at the beginning of the 20th century. The fall of the Laplacian model stimulated scientists to find a replacement for it. During the 20th century many theories were proposed including the ''planetesimal theory'' of Thomas Chamberlin and Forest Moulton (1901), ''tidal model'' of Jeans (1917), ''accretion model'' of Otto Schmidt (1944), ''protoplanet theory'' of William McCrea (1960) and finally ''capture theory'' of Michael Woolfson.〔 In 1978 Andrew Prentice resurrected the initial Laplacian ideas about planet formation and developed the ''modern Laplacian theory''.〔 None of these attempts was completely successful and many of the proposed theories were descriptive. The birth of the modern widely accepted theory of planetary formation—the solar nebular disk model (SNDM)—can be traced to the Soviet astronomer Victor Safronov. His book ''Evolution of the protoplanetary cloud and formation of the Earth and the planets'', which was translated to English in 1972, had a long-lasting effect on the way scientists think about the formation of the planets. In this book almost all major problems of the planetary formation process were formulated and some of them solved. Safronov's ideas were further developed in the works of George Wetherill, who discovered ''runaway accretion''.〔 While originally applied only to our own Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe; as of , extrasolar planets have since been discovered in our galaxy. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nebular hypothesis」の詳細全文を読む スポンサード リンク
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