how astronomers came up with the theory that the moon is a piece of a super-planet that earth was once part of or is a piece of something the earth was struck by

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how astronomers came up with the theory that the moon is a piece of a super-planet that earth was once part of or is a piece of something the earth was struck by

In: Earth Science

“a super-planet that the Earth was once part of” is a weird way of saying “the Earth”. We believe it to be the aftermath of a collision between Earth and another large stellar body because it looks to have the same composition as Earth, with the only difference being that the Moon doesn’t have shifting plates so all the rocks are as old as the oldest rocks on Earth.

Additionally, it is oddly large in comparison to Earth that it doesn’t look like a captured asteroid, which is the only other real possibility.

[Giant Impact Hypothesis](https://en.wikipedia.org/wiki/Giant-impact_hypothesis)

Paraphrased for simplicity

About 4.5 billion years ago a Mars sized planet (Theia) hit the Earth. Supporting evidence:

* Earth’s Spin and Moon’s Orbit have similar orientations
* Earth-Moon System has high angular momentum
* The moon was once molten to an unknown depth
* The moon has a small iron core
* Moon is depleted of volatile elements compared to Earth
* Evidence of similar events in other star systems
* Giant collisions consistent with leading theory for formation of the Solar System
* Lunar and Earth rock similarities imply a common origin

This evidence makes an impact theory reasonable.