Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... Debris disk around star AU Microscopii. ...

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 motion or 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. Viktor Sergeevich Safronov (Russian:Виктор Сергеевич Сафронов) (1917 - September 19, 1999) is a Soviet astronomer who was one of the first to put forward a consistent picture of how the planets formed from a disk of gas and dust around the Sun. ... In cosmogony, a protoplanet is a quasi-planetoid which is slightly larger than a planetesimal and orbits within a solar nebulas protoplanetary discs. ... In cosmogony, a protoplanet is a quasi-planetoid which is slightly larger than a planetesimal and orbits within a solar nebulas protoplanetary discs. ... The eight planets and three dwarf planets of the Solar System. ...

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 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 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-inclination moons of the Jovian planets. The Late Heavy Bombardment (LHB) was a period approximately 3. ... Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ... This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... From top: Neptune, Uranus, Saturn, and Jupiter. ... Phobos (IPA or , Greek Φόβος: Fright), is the larger and innermost of Mars two moons (the other being Deimos), and is named after Phobos, son of Ares (Mars) from Greek Mythology. ... Deimos (IPA or ; Greek Δείμος: Dread), is the smaller and outermost of Mars’ two moons, named after Deimos from Greek Mythology. ... From top: Neptune, Uranus, Saturn, and Jupiter. ...

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 Nebula from which our planetary system was formed. See meteorites and comets. A time capsule is a histori c cache of goods and/or information, usually intended as a method of communication with people in the future. ... This article or section does not cite its references or sources. ... Willamette Meteorite A meteorite is a natural object originating in outer space that survives an impact with the Earths surface without being destroyed. ... Comet Hale-Bopp Comet McNaught as seen from Swifts Creek, Victoria, Australia on 23 January 2007 A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail â€” both primarily from the effects of...

Definition of planetesimal

The word planetesimal comes from the mathematical concept infinitesimal and literally means an infinitely small planet. In mathematics, an infinitesimal, or infinitely small number, is a number that is smaller in absolute value than any positive real number. ...

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^[1] on the following definition of a planetesimal: A Small Solar System Body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets nor dwarf planets: [1] This encompasses: all minor planets apart from the dwarf planets, : the classical asteroids, (except for 1 Ceres, the... It has been suggested that minor planet be merged into this article or section. ... Comet Hale-Bopp Comet McNaught as seen from Swifts Creek, Victoria, Australia on 23 January 2007 A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail â€” both primarily from the effects of...

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 ^{[2] [3] [4]} 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. Major features of the Solar System (not to scale; from left to right): Pluto, Neptune, Uranus, Saturn, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth and its Moon, and Mars. ... Comet Hale-Bopp Comet McNaught as seen from Swifts Creek, Victoria, Australia on 23 January 2007 A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail â€” both primarily from the effects of... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... Image of the Trojan asteroids in front of and behind Jupiter along its orbital path. ...

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. Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ...

Notes and references

1. ^ http://www.mpia.de/homes/fdtp
2. ^ Morbidelli, A. Origin and dynamical evolution of comets and their reservoirs. Preprint on arXiv (pdf)
3. ^ 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 Nature article
4. ^ 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 Nature article

External links and data sources

• Discovering the Essential Universe by Neil F. Comins (2001)