Planetesimal

In cosmogony, planetesimals (pieces of planets) are objects thought to exist within solar nebulae. In nebular theory, planetesimals are thought to form from accretion within the protoplanetary disk around a newly formed star; a process of dust and ice grains colliding and sticking together, initially electrostatically and then gravitationally. Many planetesimals eventually break apart during violent collisions, but a few of the largest planetesimals can survive such encounters and continue to grow into protoplanets or even 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.

The giant impact theory proposes that Earth's Moon formed from a colossal impact of a hypothetical planetesimal, named Theia, with Earth early in the solar system's history.

Reference

• Discovering the Essential Universe by Neil F. Comins (2001)