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The Most Detailed Model Of Theia, A Mars-Sized Planet, Indicates It Formed Closer To The Sun Than Earth, Hitting Earth ~4.5Byrs Ago. The Debris Formed The Moon. Fe, Mo, Zr, O Isotope Data From Earth & Apollo Rocks Show Both Bodies Are Nearly Identical, Proving Theia Was Local. The Impact Mixed Both Worlds, & Added Metals To Earth’s Mantle.

An extra solar system planet once orbited next to Earth — and it may be the reason we have a moon | Live Science

https://www.livescience.com/space/planets/an-extra-solar-system-planet-once-orbited-next-to-earth-and-it-may-be-the-reason-we-have-a-moon

Earth may have a moon today because a nearby neighbor once crashed into us, a new analysis of Apollo samples and terrestrial rocks reveals.