Theia is a hypothesized ancient planetary-mass object in the early Solar System that according to the giant impact hypothesis collided with the Early Earth

Artist's concept of collision at HD 172555

An artist's impression of Thea's collision with Earth

around 4.533 billion years ago . According to the hypothesis, Theia was an Earth trojan about the size of Mars; if the impact were not glancing, it could have destroyed Earth. It could be responsible for creating earth's moon. It may have also tilted the earth to where it's axis is today.