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Planetesimals are solid objects thought to exist in
protoplanetary disk A protoplanetary disk is a rotating circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments i ...

protoplanetary disk
s and
debris disk A debris disk (), or debris disc (), is a of dust and debris in orbit around a . Sometimes these disks contain prominent rings, as seen in the image of on the right. Debris disks are found around stars with mature planetary systems, including ...
s. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
about 4.6 billion years ago, they aid study of its formation.


Formation

A widely accepted theory of
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
formation, the so-called planetesimal hypotheses, the Chamberlin–Moulton planetesimal hypothesis and that of
Viktor Safronov Viktor Sergeevich Safronov (russian: Ви́ктор Серге́евич Сафро́нов) (born Velikie Luki; 11 October 1917 in Russia Russia (russian: link=no, Россия, , ), or the Russian Federation, is a country spanning Eastern ...
, states that planets form from
cosmic dust Cosmic dust, also called extraterrestrial dust or space dust, is dust Dust is made of s of solid . On Earth, it generally consists of particles in the that come from various sources such as lifted by wind (an ), , and . Dust in homes ...
grains that collide and
stick
stick
to form ever-larger bodies. Once a body reaches around a kilometer in size, its constituent grains can attract each other directly through mutual
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

gravity
, enormously aiding further growth into moon-sized
protoplanet A protoplanet is a large planetary embryo that originated within a protoplanetary disc and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimal Planetesimals ...
s. Smaller bodies must instead rely on
Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physica ...

Brownian motion
or turbulence to cause the collisions leading to sticking. The mechanics of collisions and mechanisms of sticking are intricate. 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—or via the concentration and gravitational collapse of swarms of larger particles in streaming instabilities. Many planetesimals eventually break apart during violent collisions, as
4 Vesta Vesta (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...
and
90 Antiope Antiope (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any s ...
may have, but a few of the largest ones may survive such encounters and grow into protoplanets and, later, planets.


Planetesimals in the Solar System

It is generally thought that about 3.8 billion years ago, after a period known as the
Late Heavy Bombardment The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion year A year is the orbital period The orbital period is the time a given astronomical object takes ...
, most of the planetesimals within the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
had either been ejected from the Solar System entirely, into distant eccentric orbits such as the
Oort cloud The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of ...

Oort cloud
, or had collided with larger objects due to the regular gravitational nudges from the
giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
s (particularly
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
and
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
). A few planetesimals may have been captured as moons, such as Phobos and Deimos (the moons of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
) and many of the small high-
inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object. ...

inclination
moons of the giant planets. Planetesimals that have survived to the current day are valuable to science because they contain information about the
formation of the Solar System The formation and evolution of the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all indi ...
. 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 A time capsule is a historic cache of goods or information, usually intended as a deliberate method of communication with future people, and to help future archaeologists Archaeology or archeology is the study of human activity through the ...

time capsule
', and their composition might reveal the conditions in the
Solar Nebula The formation and evolution of the Solar System The Solar SystemCapitalization Capitalization ( North American English) or capitalisation ( British English) is writing a word with its first letter as a capital letter (uppercase letter) ...
from which our planetary system was formed. The most primitive planetesimals visited by spacecraft are the
contact binary Contact may refer to: Interaction Physical interaction * Contact (geology), a common geological feature * Contact lens or contact, a lens placed on the eye * Contact sport Contact sports are sport Sport pertains to any form of Competit ...
Arrokoth 486958 Arrokoth, provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permane ...
.


Definition of planetesimal

The word ''planetesimal'' comes from the mathematical concept
infinitesimal In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
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 The nebular hypothesis is the most widely accepted model in the field of cosmogony Cosmogony is any model concerning the origin of either the cosmos or the universe. Overview Scientific theories In astronomy, cosmogony refers to the st ...

planet formation
, some scientists also use the term planetesimal as a general term to refer to many
small Solar System bodies A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as foll ...
– such as
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s and
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
s – which are left over from the formation process. A group of the world's leading planet formation experts decided at a conference in 2006 on the following definition of a planetesimal:
A planetesimal is a solid object arising during the accumulation of orbiting bodies 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. In the current Solar System, these small bodies are usually also classified by dynamics and composition, and may have subsequently evolved 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.
to become comets, Kuiper belt objects or
trojan asteroid In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger one, remaining in a stable orbit approximately 60° ahead or behind the main body near one of its Lagrangian points and . Trojans can share the ...
s, 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 The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
, 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

*
Accretion (astrophysics) In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are ...
*
Disrupted planet In astronomy, a disrupted planet is a planet or exoplanet or, perhaps on a somewhat smaller scale, a planetesimal, moon, exomoon or asteroid that has been disrupted or destroyed by a nearby or passing astronomical body or object such as a star. Ne ...
*
List of interstellar and circumstellar molecules This is a list of molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physi ...
* Mesoplanet *
Planetary nebula A planetary nebula (PN, plural PNe), is a type of emission nebula An emission nebula is a nebula " from the Eagle Nebula. Evidence from the Spitzer Telescope suggests that the pillars may already have been destroyed by a supernova explos ...

Planetary nebula
* Q-PACE, a spacecraft mission to study accretion *
Ring system (astronomy) A ring system is a disc or ring orbiting an astronomical object that is composed of solid material such as cosmic dust, dust and moonlets, and is a common component of satellite system (astronomy), satellite systems around giant planets. A ring ...


Notes and references


Further reading

* ''Discovering the Essential Universe'' by Neil F. Comins (2001) * Linda T. Elkins-Tanton, et al.: ''Planetesimals – Early Differentiation and Consequences for Planets.'' Cambridge University Press, Cambridge 2017, . {{Portal bar, Physics, Astronomy, Stars, Spaceflight, Outer space, Solar System, Science *Planetesimal Planets Solar System dynamic theories