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Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. A widely accepted theory of planet formation, the so-called planetesimal hypotheses, the Chamberlin–Moulton planetesimal hypothesis and that of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals may form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk. Many planetesimals eventually break apart during violent collisions, as may have happened to 4 Vesta and 90 Antiope, but a few of the largest planetesimals may survive such encounters and continue to grow into protoplanets and later planets. It is generally believed that about 3.8 billion years ago, after a period known as the Late Heavy Bombardment, most of the planetesimals within the Solar System had either been ejected from the Solar System entirely, into distant eccentric orbits such as the Oort cloud, or had collided with larger objects due to the regular gravitational nudges from the giant planets (particularly Jupiter and Neptune). A few planetesimals may have been captured as moons, such as Phobos and Deimos (the moons of Mars), and many of the small high-inclination moons of the giant planets. Planetesimals that have survived to the current day are valuable to scientists because they contain information about the formation of the Solar System. Although their exteriors are subjected to intense solar radiation that can alter their chemistry, their interiors contain pristine material essentially untouched since the planetesimal was formed. This makes each planetesimal a 'time capsule', and their composition can tell us of the conditions in the Solar Nebula from which our planetary system was formed (see also meteorites and comets). ==Definition of planetesimal== The word ''planetesimal'' comes from the mathematical concept infinitesimal and literally means an ultimately small fraction of a planet. While the name is always applied to small bodies during the ''process'' of planet formation, some scientists also use the term planetesimal as a general term to refer to many small Solar System bodies – such as asteroids and comets – which are left over from the formation process. A group of the world's leading planet formation experts decided at a conference in 2006〔(Workshop From Dust to Planetesimals )〕 on the following definition of a planetesimal:
Bodies large enough not only to keep together by gravitation but to change the path of approaching rocks over distances of several radii start to grow faster. These bodies, larger than 100 km to 1000 km, are called embryos or protoplanets.〔Michael Perryman: ''The Exoplanet Handbook''. Cambridge University Press, 2011, ISBN 978-0-521-76559-6, .〕 In the current Solar System, these small bodies are usually also classified by dynamics and composition, and may have subsequently evolved〔 Morbidelli, A. "(Origin and dynamical evolution of comets and their reservoirs )". ''arXiv''. 〕〔 Gomes, R., Levison, H. F., Tsiganis, K., Morbidelli, A. 2005, "(Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets )". ''Nature'', 435, 466–469. 〕〔 Morbidelli, A., Levison, H. F., Tsiganis, K., Gomes, R. 2005, "(Chaotic capture of Jupiter's Trojan asteroids in the early Solar System )". ''Nature'', 435, 462–465. 〕 to become comets, Kuiper belt objects or trojan asteroids, for example. In other words, some planetesimals ''became'' other types of body once planetary formation had finished, and may be referred to by either or both names. The above definition is not endorsed by the International Astronomical Union, and other working groups may choose to adopt the same or a different definition. There is also no exact dividing line between a planetesimal and protoplanet. 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「Planetesimal」の詳細全文を読む スポンサード リンク
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