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Cloud of dust and gases collapsed under its own gravity. Because of this, the cloud flattened into a disk with the sun forming in the center of it.
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Cosmic dust get together to form planetesimals, those accrete planetesimals smaller than itself (crash into each other and the big one survives and grows in size from "consuming" the smaller one). The planetesimals grow bigger and bigger in size and become a planet.
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Metals sank through a magma ocean to the center of the Earth. Liquid metal and other elements under high pressure and temperatures forms the core in the middle of Earth.
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A planetoid the size of Mars crashes into the proto-Earth and knocks some debris off of it. The hot and molten debris stays in orbit around the Earth and cools to form the moon that we see today.
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Asteroids pounded onto planets. It is suspected that at the late heavy bombardment stage, asteroids that hit Earth delivered water and organic material to the surface.
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The warmth contained inside a body (in this case the planet) controls what sorts of surface activities, atmospheric activities, and interior activities the planet has. At this stage, the planet cools slowly, the heat diminishes, and the activities diminish to nothing.