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celestial bodies that are formed after the big bang are pulled by gravity and collide within. Earth gradually gain mass during this process
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about 500 million years after the planetary accretion, where the small particles form into the beginning stage of planet earth. Gravity pulls heavier materials to the center forming the Earth's core, and the lighter materials pushed to the outside, forming the earth's crust. Over years, the Earth traps gas on the outside of the layers, eventually forming the atmosphere. Due to planetary differentiation, the earth is separated into layers.
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Every billion years, the Earth cools down for about 100 degrees Celcius, and about 3.8-4.3 billion years ago was when the Earth became fully solid.
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About 70 million years after the Earth is formed. A large mars size body, flowing in the new solar system crashed into the orbit of Earth, causing pieces of the infant plant to break into space, and eventually, some pieces become trapped into the Earth gravity, forming the moon
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This is what completed the planetary accreation.