Theia
The name of the hypothesized protoplanet is derived from the mythical Greek titan Theia, who gave birth to the Moon goddess, Selene. This designation was proposed initially by the English geochemist Alex N. Halliday in 2000 and has become accepted in the scientific community. According to modern theories of planet formation, Theia was part of a population of Mars-sized bodies that existed in the Solar System 4.5 billion years ago. Indeed, one of the attractive features of the giant impact hypothesis is that the formation of the Moon fits into the context of the formation of the Earth: during the course of its formation, the Earth is thought to have experienced dozens of collisions with such planet-sized bodies. The Moon-forming collision would have been only one such "giant impact" and, perhaps, the last.
Read more about this topic: Giant Impact Hypothesis