A

David

Darling

accretion

Accretion is, generally speaking, a process of growth by accumulation and adhesion. Geologists use the term to describe certain modes of deposition. In the context of the origin of planetary systems (see planetary systems, formation), accretion is the process by which small particles collide and stick together to form larger objects in a protoplanetary disk. Once a large enough body has formed, its gravitational attraction greatly speeds up the accretion process.