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Celestial bodies were created through accretion, this period lasted approximately 100 million years. Large magma bodies were formed and they eventually allowed for metallic cores to be formed.
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Earth had collided with another celestial body and sent pieces of the crust into space. Gravity merged these pieces together and formed the moon. There are a multitude of different theories but this is one of the theories of greatest credbillity.
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Asteroid, meteoroids, and Comets formed the shape of the earth. The planet became molten in sphere shape and magma eventually formed the outer core of the earth.
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An abnormally large amount of Asteroids and such collided with planets in the early solar system. This theory is not "confirmed" and there are theories questioning the existence of this event.
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Molten iron around the core of the earth cooled down and froze into the present core. High pressure and High temperature helped create the shape of the core.