The Earth formed around 4.6 billion years ago, at the same time as the other planets in the Solar System. The early Earth was hot due to gravitational contraction, radioactive decay, and bombardment from meteorites. The molten Earth separated into layers based on density, with the densest materials moving to the center to form the metallic core and intermediate materials becoming the mantle. The first crust formed around 4.4 billion years ago and was rapidly recycled into the mantle due to intense heat from the early core. We know about the early Earth from zircon crystals, meteorites that originated elsewhere in the Solar System, and moon rocks.