Before revision: upright=1.3|An artist's depiction of the hypothetical impact of a planet like Theia and the Earth Theia () is a hypothesized ancient planet in the early Solar System which, according to the giant-impact hypothesis, collided with the early Earth around 4.5 billion years ago, with some of the resulting ejected debris coalescing to form the Moon.
After revision: upright=1.3|An artist's depiction of the hypothetical impact of a File:Artist's concept of collision at HD 172555.jpg| planet in the early Solar System which, according to the giant-impact hypothesis, collided with the early Earth around 4.5 billion years ago, with some of the resulting ejected debris coalescing to form the Moon.