Tiny, solid pieces of rock and metal that collect in a planetary nebula and eventually accumulate to form a planet.
A small object that orbits the Sun. Planetesimals are thought to have formed when the Solar System itself formed, and they were perhaps the building blocks from which the planets were built.
A small (diameter up to several hundred kilometers) solar-system body of the type that first condensed from the solar nebula. Planetesimals are thought to have been the principal bodies that combined to form the planets.