Every planetary system forms from a thin disk of gas and dust
in orbit around a young star. Planets grow from collisions and
mergers of smaller objects, known as planetesimals, embedded in
the disk. Protoplanets with radii of 100 km or more stir up the
remaining planetesimals along their orbits. A cascade of collisions
among rapidly moving planetesimals produces a ring of dust grains,
which slowly disappears as protoplanets grow into planets. This
process can lead to planetary systems similar to our own.