Before revision: Astronomers think the collision between Earth and Theia happened at about 4.4 to 4.45 bya ; about 0.1 billion years after the Solar System began to form.
After revision: Astronomers think the collision between Earth and Theia happened at about 4.4 to 4.45 billion years ago (bya) ; about 0.1 billion years after the Solar System began to form.