Theia is the name of a hypothetical protoplanet with the size of Mars that is thought to have collided with the proto-Earth, ejecting material into orbit to form the Moon. The hypothesis is called the Giant Impact hypothesis. The protoplanet has been named Theia after the mythical Greek goddess who gave birth to the Moon goddess, Selene. The time of impact has been determined by Hf-W dating at 4.533 Ga, 34 million years after Solar System formation. Support for the Giant Impact hypothesis comes from the similarity between the oxygen isotopic composition of terrestrial and lunar rocks and the occurrence of KREEP (K [Potassium] Rare Earth Elements P [Phosphorus])-type lunar soils and the anorthositic highlands that suggest a past molten state of the Moon – the magma ocean stage.