From Academic Kids
In cosmogony, planetesimals are objects thought to exist within solar nebulae. Planetesimals are thought to form from the coalescing (due to collisional sticking and gravity) of particles, orbiting within the accretion disc of the solar nebula; an accumulation of planetesimals leads to the formation of a protoplanetary disc, which in turn coalesce into protoplanets/planets. Some use the term to refer, in general, to objects such as asteroids and comets; other, such as Comins use the term to refer, specifically, to objects with diameters of ~10km.
It is generally argued that by about 3.8 billion years ago most of the planetesimals, within the solar system had either left orbit or coalesced into larger objects. Most of the remaining planetesimals orbit within the asteroid belt.