A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic 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, enormously aiding further growth into moon-sized protoplanets. 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 eventually break apart during violent collisions, as may have happened to 4 Vesta and 90 Antiope, 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, 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 giant 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 giant planets.
Planetesimals that have survived to the current day are valuable to scientists because they contain information about the formation 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 can tell us of the conditions in the Solar Nebula from which our planetary system was formed (see also meteorites and comets).
Definition of planetesimal 
The word planetesimal comes from the mathematical concept infinitesimal 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 – 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 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. 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 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.
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. There is also no exact dividing line between a planetesimal and protoplanet.
Notes and references 
- Savage, Don; Jones, Tammy; and Villard, Ray (1995). "Asteroid or Mini-Planet? Hubble Maps the Ancient Surface of Vesta". Hubble Site News Release STScI-1995-20. Retrieved 2006-10-17.
- Marchis, Franck; Enriquez, J. E.; Emery, J. P.; Berthier, J.; Descamps, P. (2009). "The Origin of the Double Main Belt Asteroid (90) Antiope by Component-Resolved Spectroscopy". DPS meeting #41. American Astronomical Society. Retrieved 2009-11-08.
- Workshop From Dust to Planetesimals
- Morbidelli, A. Origin and dynamical evolution of comets and their reservoirs. Preprint on arXiv
- 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
- 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