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 attrach 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.
Some scientists use the term planetesimal as a general term to refer to all objects - such as asteroids and comets - which are left over from the formation of our solar system. Others, such as Comins, use the term to refer specifically to objects with a diameter of approximately 10km.
It is generally believed that by about 3.8 billion years ago, after a period known as 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).
Most of the remaining planetesimals in our solar system orbit within the asteroid belt, the Kuiper Belt, and the Oort cloud. A few have been captured as moons, such as Phobos, Deimos, Triton, and many of the small high-inclination moons of the Jovian planets.
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