THEIA - From Wikipedia "
Theia /ˈθiːə/ (Θεία in Greek) or Orpheus (Ορφεύς) /ˈɔːrfiːəs, ˈɔːrfjuːs/ [1]) is a hypothesized ancient planet in the early Solar System that, according to the giant impact hypothesis, collided with the early Earth around 4.5 billion years ago, with the resulting debris gathering to form the Moon.[2][3]
In addition to explaining Earth's large moon, the Theia hypothesis can also explain why Earth's core is larger than would be expected for a body its size; Theia's core and mantle mixed with Earth's.[4]"