The general scientific consensus is as follows: early Earth collided with something roughly the size of Mars, chipping off a bit of our planet which would become our Moon. But there's new research to suggest the Moon was formed by a whole bunch of tiny collisions instead, over millions of years, with the fragments eventually forming the Moon we see today.
The researchers say this would explain why the Moon appears to be composed largely of Earth-like material, rather than a mix of Earth and another planet.
Dr Raluca Rufu and team conducted numerical simulations of large planetary bodies (but not giant ones) impacting the early Earth, or "proto-Earth". In the simulations, the impacts produce disks of debris, many of which are composed of mostly Earth, not impactor, material.
After each impact, the debris disks bunch together to form a "moonlet" that the researchers suggest eventually migrates outwards, and joins with a growing Moon. It would take about 20 of these moonlet-forming collisions to assemble the Moon.
"Moonlet-forming impacts between the proto-Earth and large bodies were sufficiently common in the early inner Solar System to produce the Moon and match observational constraints," the research says. "A Moon assembled through multiple impacts implies that the Moon formed over many millions of years, rather than in a geologic instant, and that the Earth and Moon's interiors may be less well mixed than in a giant-impact scenario, potentially preserving a record of this period of bombardment".