Theories have been developed regarding Moon formation, the formation of the Moon, as well as other moons, which likely result from various processes. The giant-impact hypothesis is currently the standard model of the formation of the Moon, that sometime in the very early history of the solar system, a Mars-sized planet (termed Theia) struck the Earth sending debris into space which eventually coalesced into a circumplanetary disk. This explains why lunar material is strikingly similar to Earth material in isotope abundances (isotope signature) and radioactivity characteristics (though the Moon has a lower level of volatiles, which also requires explanation). A current issue with the model is that lunar material has not shown signs of the different material expected from an impacting planet. Other recent variations and hypotheses regarding an explanation:
Considered less likely:
Each model has its problems.
Other solar system moons may have formed through one or more of these processes, with an additional possibility of capture of an asteroid. Suitable capture methods require some coincidence, e.g., the simultaneous nearby presence of a third body, but it is seriously considered, for example, for Neptune's moon, Triton, due to its a retrograde orbit. Capture seems unlikely for the Moon because of its Earth-like isotope signature.