A Modern Protoplanet Named for the Mother of the Moon

Theia's name has been given a second life in modern planetary science, attached not to a myth but to a hypothesis about the literal origin of the Moon. The giant-impact hypothesis, the leading scientific explanation for how Earth's Moon formed, proposes that a Mars-sized protoplanet collided with the early Earth roughly 4.5 billion years ago, and the debris thrown into orbit by that collision coalesced to form the Moon; planetary scientists who developed and refined this model in the late twentieth century named the hypothetical impactor Theia, a deliberate and fitting borrowing, since the mythological Theia was the mother of Selene, the Moon, making the name of the Moon's mythic parent an apt label for the object whose destruction is thought to have physically produced the Moon itself.

Theia the protoplanet has never been directly observed and cannot be, since the hypothesis holds it was destroyed and absorbed in the very collision that created the Moon, its former existence inferred indirectly from the chemical composition of lunar rock samples returned by the Apollo missions, whose isotopic similarities to Earth rock have themselves generated ongoing debate among planetary scientists about exactly how much of the Moon's material came from Theia rather than from Earth itself. The naming shows a small but genuine pattern in modern scientific practice, in which astronomers and planetary scientists reach for the same store of Greek mythological names ancient poets used for the sky, giving Theia a documented career in twenty first century cosmology considerably more prominent than any cult she held in antiquity.