A protoplanetary disk is a rotating disk of gas and dust surrounding a newly formed star, known as a protostar. These disks are crucial in the formation of planets, as they provide the material needed for planetesimals to coalesce and grow into larger bodies. The composition of protoplanetary disks typically includes hydrogen, helium, and various ices and minerals. Over time, the gravitational forces within the disk can lead to the formation of planets, moons, and other celestial objects, marking the early stages of planetary system development.