Planetesimals (/ˌplænɪˈtɛsɪməlz/) are solid objects thought to exist inprotoplanetary disks anddebris disks. Believed to have formed in theSolar System about 4.6 billion years ago, they aid study ofits formation.
A widely accepted theory ofplanet formation, the planetesimal hypothesis ofViktor Safronov, states that planets form fromcosmic dust grains that collide andstick to form ever-larger bodies. Once a body reaches around a kilometer in size, its constituent grains can attract each other directly through mutualgravity, enormously aiding further growth into moon-sizedprotoplanets. Smaller bodies must instead rely onBrownian motion or turbulence to cause the collisions leading to sticking. The mechanics of collisions and mechanisms of sticking are intricate.[2][3] 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 instreaming instabilities.[4] Many planetesimals eventually break apart during violent collisions, as4 Vesta[5] and90 Antiope may have,[6] but a few of the largest ones may survive such encounters and grow into protoplanets and, later, planets.
It has been inferred that about 3.8 billion years ago, after a period known as theLate Heavy Bombardment, most of the planetesimals within theSolar System had either been ejected from the Solar System entirely, into distant eccentric orbits such as theOort cloud, or had collided with larger objects due to the regular gravitational nudges from thegiant planets (particularlyJupiter andNeptune). A few planetesimals may have been captured as moons, such asPhoebe (a moon ofSaturn) and many other small high-inclination moons of the giant planets.
Planetesimals that have survived to the current day are valuable to science because they contain information about theformation 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 might reveal the conditions in theSolar Nebula from which our planetary system was formed. The most primitive planetesimals visited by spacecraft are thecontact binaryArrokoth.[7]
The wordplanetesimal is derived from the wordinfinitesimal and means an ultimately small fraction of a planet.[citation needed]
While the name is always applied to small bodies during theprocess ofplanet formation, some scientists also use the term planetesimal as a general term to refer to manysmall Solar System bodies – such asasteroids andcomets – which are left over from the formation process. A group of the world's leading planet formation experts decided at a conference in 2006[8] 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 bygas 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.[9]
In the current Solar System, these small bodies are usually also classified by dynamics and composition, and may have subsequently evolved[10][11][12]to become comets,Kuiper belt objects ortrojan asteroids, for example. In other words, some planetesimalsbecame 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 theInternational Astronomical Union, and other working groups may choose to adopt the same or a different definition. The dividing line between a planetesimal and protoplanet is typically framed in terms of the size and the stages of development that the potential planet has already gone through: planetesimals combine to form a protoplanet, and protoplanets continue to grow (faster than planetesimals).[13][14][15]
