Was the Moon-Forming Protoplanet 'Theia' a Neighbor of Earth?

Theia crashed into earth and formed the moon, the theory goes. But then where did Theia come from? The lead author on a new study says "The most convincing scenario is that most of the building blocks of Earth and Theia originated in the inner Solar System. Earth and Theia are likely to have been neighbors." Though Theia was completely destroyed in the collision, scientists from the Max Planck Institute for Solar System Research led a team that was able to measure the ratio of tell-tale ...