a name="Caption"> Caption: "Animation of Theia possibly forming in Earth's L5 point and then drifting into impact event with the earth. The animation progresses in one-year steps (before impact) making the Earth appear NOT to move. The view is of the South Pole." (Somewhat edited.)

Theia is the posited protoplanet whose impact causes the formation of the Moon in the giant impact hypothesis.

Features:

1. The L4 and L5 points are 2 of the 5 Lagrangian points where a small body can be in a equilibrium position relative two much more massive mutually orbiting bodies.

2. L4 and L5 points are stable in that a small body put there will tend to stay there (Wikipedia: Lagrangian point: Stability). If one of the massive bodies is much smaller than the other, the L4 and L5 points lie nearly on the orbit of the smaller one about the larger one at leading and trailing positions of ∼ 60° along the orbit with these angles subtended at the Sun.

3. The Trojan asteroids of Jupiter orbit the Sun nearly on Jupiter's orbit near Jupiter's L4 and L5 points with the Sun.

   The Trojans stay in these orbits quasi-perpetually.

4. In the Lagrangian point theory of the origin of Theia, Theia forms in either one or the other of the Earth's L4 and L5 points with the Sun.

   Somehow gravitational perturbations destablize Theia and send it off to become the giant impactor of the giant impact hypothesis of the origin of the Moon.

5. Although we don't know yet exactly when the giant impact occurred, in the Lagrangian point theory the giant impact was circa 4.5 Gyr ago or only of order 0.1 Gyr after the solar system formation when large protoplanets were common.

6. In Greek mythology, Theia (whose name means goddess) is the Titan goddess who was the mother of the Moon goddess Selene.