Planetesimal

Once a body reaches around a kilometer in size, its constituent grains can attract each other directly through mutual gravity, enormously aiding further growth into moon-sized protoplanets.

[4] Many planetesimals eventually break apart during violent collisions, as 4 Vesta[5] and 90 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 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).

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.

[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 or trojan asteroids, for example.