How can tiny dust grains help build a planet?

Earth did not begin as a finished ball waiting beside the Sun. The young Sun was surrounded by a disk of gas and dust. Material in that disk orbited the star, and encounters between pieces helped build larger bodies.

At first, very small grains could stick when they collided gently. Some collisions broke clumps apart instead. Gas affected how particles moved, and growing concentrations of solids helped larger bodies form. Once bodies became massive enough, their gravity helped them gather more material. These early building blocks are called planetesimals.

Over many collisions, some bodies grew into young planets. Impacts delivered material and energy, so the growing worlds could become very hot. Close to the Sun, rocky material survived where many ices could not. Farther out, colder conditions allowed more ice to join the construction. Some large cores gathered thick envelopes of gas before much of the disk's gas dispersed.

Growth was neither neat nor identical everywhere. Bodies collided, scattered one another and sometimes changed orbits. Asteroids and comets preserve clues to material left over from these events. Telescopes also reveal disks around young stars today. By connecting those observations with rocks from our own solar system, scientists investigate how countless small pieces became whole worlds.