MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Planetary formation. Artwork of an Earth-like planet forming in the early solar system. It is thought that the planets formed from a rotating disc of material that formed around, and at the same time as, the Sun (upper left), around 4. This protoplanet grew in size as it gathered up the surrounding material (such as the rocky debris seen) by gravitational attraction. Impact craters from this early history are still seen on the Moon, but have eroded away from the surface of the Earth. The energies of the impacts have turned the planet molten, allowing heavy elements (like iron) to sink to the core and a lighter silicate crust to form upon cooling.
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