system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did they come up with the theory that a Mars sized planet, Theia, collided with early Earth creating the moon?

Watching The Universe on H2 and they told about a Mars sized planet, Theia, colliding with Earth and creating the moon. Is there any proof of this? If not what lead to that theory?
gpt
It's impossible for a satellite the size of the Moon to be caught in a planet the size of Earth's gravity.  Plus the material make up of the moon is similar to that of our crust/mantle.

Couple that with lots of simulations showing that another planet had to swipe by the Earth a couple times to put up enough material without destroying them both, and it's the best theory we've found thus far.