Several mechanisms have been suggested for my formation, and where I came from but nobody knows for certain, not even me.
My formation is believe to have occurred 4.527 ± 0.010 billion years ago, about 30–50 million years after the origin of the Solar System
Fission hypothesis
Early speculation proposed that I broke off from the Earth's crust because of centrifugal forces, leaving a basin – presumed to be the Pacific Ocean – behind as a scar. This idea, however, would require too great an initial spin of the Earth; and, even had this been possible, the process should have resulted in my orbit following Earth's equatorial plane. This is not the case.
Early speculation proposed that I broke off from the Earth's crust because of centrifugal forces, leaving a basin – presumed to be the Pacific Ocean – behind as a scar. This idea, however, would require too great an initial spin of the Earth; and, even had this been possible, the process should have resulted in my orbit following Earth's equatorial plane. This is not the case.
Capture hypothesis
Other speculation has centered me being formed elsewhere and subsequently being captured by Earth's gravity. However, the conditions believed necessary for such a mechanism to work, such as an extended atmosphere of the Earth in order to dissipate my energy, are improbable.
Co-formation hypothesis
The co-formation hypothesis proposes that the Earth and meformed together at the same time and place from the primordial accretion disk. I would have formed from material surrounding the proto-Earth, similar to the formation of the planets around the Sun. Some suggest that this hypothesis fails adequately to explain the depletion of metallic iron in me.
A major deficiency in all these hypotheses is that they cannot readily account for the high angular momentum of the 'Earth–Moon' system.
Giant Impact hypothesis
The prevailing hypothesis today is that the my system with the Earth formed as a result of a giant impact. A Mars-sized body (labelled "Theia") is believed to have hit the proto-Earth, blasting sufficient material into orbit around the proto-Earth to form me through accretion. As accretion is the process by which all planetary bodies are believed to have formed, giant impacts are thought to have affected most if not all planets. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the 'Earth–Moon' system, as well as the small size of the lunar core.Unresolved questions regarding this theory concern the determination of the relative sizes of the proto-Earth and Theia and of how much material from these two bodies formed me!
