A body of rock in space, smaller than a planet. Scientists study planetesimal images from telescopes in Earth and in space, adhering to the modern theory that planetesimals were the building blocks of planets.

The Sun and the planets are thought to have formed from a rotating dust cloud generated by a supernova explosion. On condensation, this cloud formed a central sun and a rotating disc, the material of which separated into rings of dust grains that began to stick together. Larger and larger clumps formed in each ring and eventually collected into bodies the size of present-day asteroids, or planetesimals. After numerous collisions, these bodies eventually formed the nucleus of the various planets of our Solar System.