Theia is a hypothetical planet that scientists believe existed in the early solar system. It is thought to have been about the size of Mars and is best known for its role in the formation of the Earth. According to the giant impact hypothesis, Theia collided with the young Earth, leading to the creation of the Moon and significantly altering Earth's composition. The name "Theia" comes from Greek mythology, where she is the goddess of sight and the mother of the Moon goddess, Selene. This connection highlights the importance of Theia in understanding the origins of both the Earth and its natural satellite, the Moon.