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When our Solar System began to settle into its current state 4.5 billion years ago, gravity, over millions of years, began to pull rocks and dust together which eventually formed into Earth.
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Directly following the formation of proto-Earth, it collided with another planet similar to the size of Mars, named Theia. The interaction between these two planets produced immense amounts of energy and sent molten debris into orbit: eventually developing into the Moon.
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Gradually over time the Earth went through another phase. In this stage of the Earths life, denser elements bonded together and sank to Earths center where it became the Earths core while lighter elements began to cool on the surface: forming the crust.
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As the surface of the Earth solidified from its original state, comets continued to impact the Earth. Study shows that comets are predominantly composed of water and is a main ingredient of a comet's nucleus. Now that the surface of the Earth was no longer molten rock, later comets were able to eject its water into the atmosphere which eventually rained back down.
