An Etymological Dictionary of Astronomy and Astrophysics

Any of numerous small solid bodies in a → protoplanetary disk that in some cases clump together to form → planets but in other cases remain relatively small and become → asteroids and → comets. Similarly, → Kuiper Belt Objects are probably the remnants of the planetesimals that formed the planets.

Etymology (EN): From → planet + (infinit)esimal, → infinitesimal.

Etymology (PE): Xordesayyâré, from xordé “small, minute; crumbs,” from
xord “minute, little, small”
(from Mid.Pers. xvart, xôrt “small, insignificant;” Av. ādra- “weak, dependent;” Skt. ādhrá- “small, weak, poor,” nādh “to be oppressed;” Gk. nothros “sluggish;” PIE base *nhdhro-) + sayyâré, → planet.