Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. They are formed from grains of cosmic dust that collide and stick together. They are the building blocks that accumulate to form planets during the process of planetary formation.
The Oort Cloud is a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances of up to 100,000 AU. It is believed to be the source of all long-period comets.