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 hypothesis of
Viktor Safronov, states that planets form out of 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, aiding further growth into moon-sized protoplanets enormously. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motionor turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals can 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 may eventually break apart during violent collisions, but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets and later planets.
It is generally believed that by about 3.8 billion years ago, after a period known as the
Late Heavy Bombardment, most of the planetesimals within the solar systemhad 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 Jovian 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- inclinationmoons of the Jovian planets.
Planetesimals that have survived to the current day are valuable to scientists because they contain information about the birth of our 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 Nebulafrom which our planetary system was formed. See meteorites and comets.
Definition of planetesimal
The word "planetesimal" comes from the mathematical concept
infinitesimaland 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 on a conference in 2006 [ [http://www.mpia.de/homes/fdtp Workshop From Dust to Planetesimals ] ] on the following definition of a planetesimal:
"A planetesimal is a solid object arising during the accumulation of planets whose internal strength is dominated by self-gravity and whose orbital dynamics is not significantly affected by gas drag. This corresponds to objects larger than approximately 1 km in the solar nebula."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." [http://arxiv.org/abs/astro-ph/0512256 Preprint on 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 [http://www.nature.com/nature/journal/v435/n7041/abs/nature03676.html Nature article] ] [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 [http://www.nature.com/nature/journal/v435/n7041/abs/nature03540.html Nature article] ] to become comets, Kuiper belt objects or trojan asteroids, for example. In other words, some planetesimals "became" other populations once planetary formation had finished, and may be referred to by either or both names.
It should be noted that 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.
Notes and references
External links and data sources
Discovering the Essential Universe" by Neil F. Comins( 2001)
Wikimedia Foundation. 2010.
Look at other dictionaries:
Planetesimal — Saltar a navegación, búsqueda Disco protoplanetario HH 30 en la constelación de Tauro Los planetesimales son objetos sólidos que se estima que existen en los discos protoplanetarios. En esa primitiva nebulosa de gases y polvo en forma de disco,… … Wikipedia Español
Planetesimal — Planétésimal Un planétésimal est un petit corps solide de 5 à 10 kilomètres de diamètre et d une masse de l ordre de mille milliards de tonnes. Il se forme au sein d une nébuleuse de débris et entre habituellement dans le processus de formation d … Wikipédia en Français
Planetesimal — Planetesimal, kleiner Körper des Urplanetensystems aus kondensierter Materie, der Grundlage der Planetenentstehung war. Man geht davon aus, dass die Planeten aus Verdichtungen in einer Gaswolke entstanden, die nach der Entstehung der Sonne um… … Universal-Lexikon
planetesimal — [plan΄ə tes′i məl, plan΄ətezi məl] n. [ PLANET + (INFINIT)ESIMAL] in various theories about the origin of the solar system, any of the small, cold masses of rock or ice that merged to form a planet … English World dictionary
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planetesimal — /plan i tes euh meuhl/, Astron. n. 1. one of the small celestial bodies that, according to one theory (planetesimal hypothesis), were fused together to form the planets of the solar system. adj. 2. of or pertaining to a planetesimal or… … Universalium
planetesimal — /plænəˈtɛsəməl/ (say planuh tesuhmuhl) adjective 1. of or relating to minute bodies in the solar system or in similar systems, which, according to the planetesimal hypothesis, move in planetary orbits and gradually unite to form the planets and… … Australian English dictionary
Planetesimal — Künstlerische Darstellung von Planetesimalen Planetesimale sind die Vorläufer und Bausteine von Planeten. Die Bezeichnung Planetesimal wurde 1904 von dem US amerikanischen Geologen Thomas Chrowder Chamberlin ins Englische eingeführt. Er hat sie… … Deutsch Wikipedia
planetesimal — n. any of a vast number of minute planets or planetary bodies. Phrases and idioms: planetesimal hypothesis the theory that planets were formed by the accretion of planetesimals in a cold state. Etymology: PLANET, after infinitesimal … Useful english dictionary
planetesimal — noun Etymology: planet + esimal (as in infinitesimal) Date: 1903 any of numerous small solid celestial bodies that may have existed at an early stage of the development of the solar system … New Collegiate Dictionary