World Library  
Flag as Inappropriate
Email this Article

Planetesimal

Article Id: WHEBN0000192500
Reproduction Date:

Title: Planetesimal  
Author: World Heritage Encyclopedia
Language: English
Subject: Nebular hypothesis, Asteroid, Circumstellar disk, Planet, Planetary system
Collection: Circumstellar Disks, Planets, Solar System Dynamic Theories
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Planetesimal

Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.

Debris disks detected in HST archival images of young stars, HD 141943 and HD 191089, using improved imaging processes (24 April 2014).[1]

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[2] and 90 Antiope,[3] 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).

Contents

  • Definition of planetesimal 1
  • See also 2
  • Notes and references 3
  • Further reading 4

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[4] 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.

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.[5]

In the current Solar System, these small bodies are usually also classified by dynamics and composition, and may have subsequently evolved[6][7][8] 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.

See also

Notes and references

  1. ^ Harrington, J.D.; Villard, Ray (24 April 2014). "RELEASE 14-114 Astronomical Forensics Uncover Planetary Disks in NASA's Hubble Archive".  
  2. ^ Savage, Don; Jones, Tammy; Villard, Ray (1995). "Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta". Hubble Site News Release STScI-1995-20. Retrieved 2006-10-17. 
  3. ^ Marchis, Franck; Enriquez, J. E.; Emery, J. P.; Berthier, J.; Descamps, P. (2009). The Origin of the Double Main Belt Asteroid (90) Antiope by Component-Resolved Spectroscopy. DPS meeting #41. American Astronomical Society. Retrieved 2009-11-08. 
  4. ^ Workshop From Dust to Planetesimals
  5. ^ Michael Perryman: The Exoplanet Handbook. Cambridge University Press, 2011, ISBN 978-0-521-76559-6, [2], p. 226, at Google Books.
  6. ^ Morbidelli, A. "Origin and dynamical evolution of comets and their reservoirs". arXiv.
  7. ^ 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.
  8. ^ 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.

Further reading

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.