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Planetesimals are objects within the solar nebulae of protostars. Planetesimals 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 or planets. Some use the term to refer, in general, to objects such as asteroids and comets; other specifically to objects with diameters of ~10km.