Planetesimals are the precursors and building blocks of planets . They arise in the protoplanetary disk of a later planetary system , e.g. B. in the solar nebula , the forerunner of the solar system , from condensation products and their progressive adhesion ( coagulation ) to objects a few millimeters in size. By accretion , they then grow to a diameter of a few kilometers.
In the gas disk, particles of the interstellar dust , such as presolar minerals in the solar nebula, act as condensation nuclei and catalysts for chemical reactions . They form droplets and solid particles through condensation.
If such condensates collide at low speed, they stick together due to surface adhesion or chemical bonds . As the agglomeration progresses, this coagulation leads to larger agglomerations and thus to the first planetesimals.
Larger planetesimals grow increasingly with the help of their gravity . The bodies collect only small amounts of dust and unite with other planetesimals to form objects like rubble piles . When a certain mass is reached , the loosely bound planetesimal clusters are pressed together by gravity to form a uniform object which, when the mass is sufficiently high, heats up and becomes liquid and thus develops into a protoplanet .
Computer simulations have shown that the process of agglomeration within a protoplanetary disk takes place in a relatively short period of time: in just 100,000 years the planetesimals of the early solar system could evolve into planetary bodies the size of the Earth's moon or the planet Mars .
In 1900 Chamberlin, together with the American astronomer Forest Ray Moulton , put forward a theory according to which the planetary system was formed from material that a star had snatched from the sun by tidal forces during a very close encounter . This Chamberlin-Moulton theory is now obsolete. It is sometimes also referred to as the planetesimal hypothesis . After 1940 the Soviet geophysicist Otto Juljewitsch Schmidt carried on the ideas in this area; after him mainly his pupil Viktor Sergejewitsch Safronow (1917-1999).
- Joachim Gürtler, Johann Dorschner: The solar system. Barth, Leipzig / Berlin / Heidelberg 1993, ISBN 3-335-00281-4 .
- Linda T. Elkins-Tanton et al .: Planetesimals - Early Differentiation and Consequences for Planets. Cambridge University Press, Cambridge 2017, ISBN 978-1-107-11848-5 .
- Anita Ehlers (trans.): Lexicon of Astronomy. Steiger, Augsburg 1999, ISBN 3-89652-191-8 , p. 140.
- J. Gürtler, J. Dorschner: The solar system. 1993, p. 229.
- Rolf Sauermost (ed.): Encyclopedia of Astronomy. The great encyclopedia of space exploration in two volumes. Volume 1, Herder, Freiburg / Basel / Vienna 1989, ISBN 3-451-21632-9 , p. 148 and Volume 2, p. 114.
- David R. Oldroyd: The Biography of the Earth. On the history of science in geology. Zweiausendeins, Frankfurt am Main 1998, ISBN 3-86150-285-2 , p. 408.