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The Moon might have been created by the impact between the Earth and a Mars-sized body named Theia. Theia no longer exists, so when and how was it discovered? What clues led to the Theia hypothesis? What is the story of Theia's discovery?

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    $\begingroup$ You might have this backwards. We didn't discover a Mars-sized body, and deduce that it must have impacted on the early Earth. We discovered (evidence that suggests) a giant impact, and deduced the existence of a Mars-sized body. That is we didn't discover Theia, we discovered the impact. $\endgroup$
    – James K
    Aug 18 at 16:05
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One link away from the Wikipedia article you mention there is a reasonably comprehensive list:

  • Earth's spin and the Moon's orbit have similar orientations.

  • The Earth–Moon system contains an anomalously high angular momentum. Meaning, the momentum contained in Earth's rotation, the Moon's rotation, and the Moon revolving around Earth is significantly higher than the other terrestrial planets. A giant impact may have supplied this excess momentum.

  • Moon samples indicate that the Moon was once molten down to a substantial, but unknown, depth. This may have required more energy than predicted to be available from the accretion of a body of the Moon's size. An extremely energetic process, such as a giant impact, could provide this energy.

  • The Moon has a relatively small iron core. This gives the Moon a lower density than Earth. Computer models of a giant impact of a Mars-sized body with Earth indicate the impactor's core would likely penetrate Earth and fuse with its own core. This would leave the Moon with less metallic iron than other planetary bodies.

  • The Moon is depleted in volatile elements compared to Earth. Vaporizing at comparably lower temperatures, they could be lost in a high-energy event, with the Moon's smaller gravity unable to recapture them while Earth did.

  • There is evidence in other star systems of similar collisions, resulting in debris discs.

  • Giant collisions are consistent with the leading theory of the formation of the Solar System.

  • The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin.

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Theia has not been discovered. The giant impact hypothesis is, as the name suggests, a scientific hypothesis -- an idea that proposes a tentative explanation about a phenomenon or a narrow set of phenomena observed in the natural world (emphasis mine). The giant impact hypothesis is by far the most widely accepted hypothesis regarding the formation of the Moon. However, several outstanding issues with regard to the hypothesis remain that need to be addressed, which is why "hypothesis" remains the best term to describe the concept of a giant impact that resulted in the formation of the Moon.

If such a giant impact did occur, why not give the impactor a name? The name Theia was first proposed by Alex N. Halliday in the peer-reviewed scientific journal article Terrestrial accretion rates and the origin of the Moon published in 2000.

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