Planetesimal
Overview
 
Planetesimals are solid objects thought to exist in protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...

s and in debris disk
Debris disk
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...

s.

A widely accepted theory of planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 formation, the so-called planetesimal hypothesis of Viktor Safronov
Viktor Safronov
Viktor Sergeevich Safronov was a Soviet astronomer who put forward the low-mass-nebula model of planet formation, a consistent picture of how the planets formed from a disk of gas and dust around the Sun.-Biography and legacy:Safronov graduated from Moscow State University Department of...

, states that planets form out of cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

 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 protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...

s.
Unanswered Questions
Encyclopedia
Planetesimals are solid objects thought to exist in protoplanetary disk
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...

s and in debris disk
Debris disk
A debris disk is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...

s.

A widely accepted theory of planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

 formation, the so-called planetesimal hypothesis of Viktor Safronov
Viktor Safronov
Viktor Sergeevich Safronov was a Soviet astronomer who put forward the low-mass-nebula model of planet formation, a consistent picture of how the planets formed from a disk of gas and dust around the Sun.-Biography and legacy:Safronov graduated from Moscow State University Department of...

, states that planets form out of cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

 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 protoplanet
Protoplanet
Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...

s. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion
Brownian motion
Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...

 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 eventually break apart during violent collisions, as may have happened to 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 and 90 Antiope
90 Antiope
The most remarkable feature of Antiope is that it consists of two components of almost equal size , making it a truly "double" asteroid. Its binary nature was discovered on 10 August 2000 by a group of astronomers using adaptive optics at the Keck Telescope on Mauna Kea. The "secondary" is...

, 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 about 3.8 billion years ago, after a period known as the Late Heavy Bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...

, most of the planetesimals within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 had either been ejected from the Solar System entirely, into distant eccentric orbits such as the Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...

, or had collided with larger objects due to the regular gravitational nudges from the Jovian planets (particularly Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

). A few planetesimals may have been captured as moons, such as Phobos
Phobos (moon)
Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...

 and Deimos
Deimos (moon)
Deimos is the smaller and outer of Mars's two moons . It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is '.-Discovery:Deimos was discovered by Asaph Hall, Sr...

 (the moons of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

), and many of the small high-inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

 moons of the Jovian planets.

Planetesimals that have survived to the current day are valuable to scientists because they contain information about the formation of the Solar System
Formation and evolution of the Solar System
The formation and evolution of the Solar System is estimated to have begun 4.568 billion years ago with the gravitational collapse of a small part of a giant molecular cloud...

. 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
Time capsule
A time capsule is an historic cache of goods or information, usually intended as a method of communication with future people and to help future archaeologists, anthropologists, or historians...

', and their composition can tell us of the conditions in the Solar Nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...

 from which our planetary system was formed (see also meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...

s and comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s).

Definition of planetesimal

The word planetesimal comes from the mathematical concept infinitesimal
Infinitesimal
Infinitesimals have been used to express the idea of objects so small that there is no way to see them or to measure them. The word infinitesimal comes from a 17th century Modern Latin coinage infinitesimus, which originally referred to the "infinite-th" item in a series.In common speech, an...

 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
Small Solar System body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...

 - such as asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s and comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s - which are left over from the formation process. A group of the world's leading planet formation experts decided on a conference in 2006 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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

. This corresponds to objects larger than approximately 1 km in the solar nebula.


In the current Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, these small bodies are usually also classified by dynamics and composition, and may have subsequently evolved
to become comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s, Kuiper belt objects
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 or trojan asteroid
Trojan asteroid
The Jupiter Trojans, commonly called Trojans or Trojan asteroids, are a large group of objects that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each Trojan librates around one of the planet's two Lagrangian points of stability, and , that respectively lie 60° ahead...

s, 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.

The above definition is not endorsed by the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

, 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

  • Asteroid
    Asteroid
    Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

  • Dwarf planet
    Dwarf planet
    A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...

  • Fusor (astronomy)
    Fusor (astronomy)
    A fusor is a term proposed to the IAU by Gibor Basri, Professor of Astronomy at the University of California, Berkeley, to help clarify the nomenclature of celestial bodies...

  • Mesoplanet
    Mesoplanet
    Mesoplanet is a term coined by Isaac Asimov to refer to planetary bodies with sizes smaller than Mercury but larger than Ceres. Assuming "size" is defined by linear dimension , mesoplanets should be approximately 1,000 km to 5,000 km in diameter...

  • Planetar (astronomy)
    Planetar (astronomy)
    Planetar is a term used in astronomy that refers to one of two things:* Brown dwarfs — objects intermediate in size between planets and stars — but having formed similarly to planets....

  • Protoplanet
    Protoplanet
    Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide...


External links and data sources

  • Discovering the Essential Universe by Neil F. Comins (2001)
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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