it is thought that another earth-like planet but half of earth's size,
Theia collided with earth as it was developing some 4.5 billion years ago and the heat of the impact creating magma oceans and ejected a lot of the mass of Theia and this ejected debris coalesced to form the
moon which is thought to consist of 80% Theia and 20% earth, but with much less iron as this is heavy and was retained on earth, and much less low atomic weight elements such as hydrogen as the heat would have boiled these off.
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