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This is how the Earth formed. Planetary accretion happened over the course of 100 million years. Solar winds swept away lighter elements like helium and hydrogen, which made Earth form into a small and rocky planet.
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Early on in Earth's development, an asteroid about the size of Mars collided with the Earth. This catapulted pieces of both the Earth and the asteroid into space, where gravity pulled it together to form the moon. The Earth's gravitational pull caused the newly formed moon to start orbiting it.
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The collision with the asteroid left the Earth as a sea of magma. Since the entire planet was liquid, the denser substances sank and the lighter substances rose, thus forming the layers that modern-day Earth has.
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The Late Heavy Bombardment was when the Earth and the moon were constantly hit by failed planets and smaller asteroids. This period ended around 3.8 billion years ago.
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Since its formation, the Earth has been cooling at a rate of 100 degrees Celsius every billion years. But 3.5 billion years ago is the earliest that scientists can prove that Earth had cooled enough to solidify.
