This drawing illustrates how forming moons drew gas to themselves.
Image from: The New Solar System
The Co-Formation Theory
The co-formation theory explains the origin of the moon as an object which formed out of the initial cloud of gas and dust
at the same time and roughly the same place as the Earth. As shown in this picture, while they were forming in the solar nebula, the beginning of the moons-to-be (called protomoons) drew material
to themselves from the cloud of gas and dust around them.
Because the proto-moon was so close to the proto-earth the gas out of which they both formed ought to be very similar, composed mostly of something which would form rock.
The co-formation theory explains why the moon appears in the location it does but it does not explain the evidence that the Earth and Moon do not appear to be made of the same stuff.
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