How A Rocky Planet Forms. Keychain

An artist's rendition of how a rocky planet forms. As a rocky planet forms, the planet-forming material gathers in a process known as accretion. It grows larger in size, and increases in temperature, along with the pressure at its core. The energy from this initial planet forming process causes the planet's elements to heat up and melt. Upon melting, layers form and separate. The heavier elements sink to the bottom, the lighter ones float to the top. This material then separates into layers as it cools, which is known as differentiation. A fully formed planet slowly emerges, with an upper layer known as the crust, the mantle in the middle, and a solid iron core.