Solar systems begin in dusty gas clouds called nebulae that are found between stars. The dust is composed of elements like iron and carbon. Nebula gases include hydrogen and helium.
Within a nebula, dust particles collide and stick together to form larger particles. This process is called accretion. The particles grow from pebbles, to rocks, to boulders. The larger particles are called planetesimals.
3. Rocky and Gaseous Planet Formation
Rocky planetesimals near the Sun grew by accretion to become the rocky planets—Mercury, Venus, Earth and Mars. In the colder outer parts of the solar system, frozen gases accreted to rocky cores to form the gas giant planets—Jupiter, Saturn, Uranus and Neptune.