The mainstream view for a long time has been that the Moon was formed when a Mars-sized body (“Theia”) collided with the proto-Earth and knocked part of it into orbit. There it mixed with material from Theia to form the Moon. Some authors, however, still favour the idea that the Moon was ejected from the proto-Earth, as George Darwin had proposed long ago.
In a new paper, I argue likewise that the Moon was ejected in this way. I propose that the large low-velocity provinces were involved, two huge masses sitting on the core-mantle boundary. I suggest three possible energy sources for the ejection and no one will be surprised when I say the Hubble luminosity is the most likely one. I have renamed it as the lambda luminosity to connect with my shell universe model (and also because no one was using the Hubble luminosity term!)
https://link.springer.com/article/10.1007/s11631-025-00834-2
A well researched and informative read. Comprehensive and convincing! It’s not a subject I have thoroughly explored so my comments are limited to an admiration of the scope of the work that went into it. Nicely done, Matt.
From the few models I have seen, including yours, I don’t recall seeing a scenario where earth and moon form from the same protoplanetary material. In similar orbit around sol but out-of-phase, to inevitably eventually coincide in vicinity and gravitational capture of each other. Is there an obvious reason that this scenario is excluded?
Thanks, Francois! I have to give a lot of credit to a very tough reviewer who just forced me to make it better. I learned quite a lot as a result.
The theory you mention is sometimes called the accretion theory, the co-accretion theory or the co-formation theory. The main criticism seems to be that it can’t explain why the angular momentum of the Earth-Moon system is as high as it is. If you think of two planetary bodies forming side by side or even spaced apart in a similar orbit, what would cause the small one to go into a fast orbit around the larger one? The other main criticism seems to be that the iron content of the Moon is far less than the Earth’s. Why should that be if they formed from similar clouds of material? In my model the difference is explained with the Moon being formed mostly of mantle rock rather than core material. The giant impactor theory does the same.
Here is a nice video on the topic of Moon theories, a bit out of date but still informative:
https://www.youtube.com/watch?v=AKqq4MF3fns&t=483s