Hunting A Hypothetical Planet

NASA's STEREO mission studies solar activity using two probes, one each on opposite sides of the Sun. Scientists, however, intend to use the probes to do a little bonus work.

The leading current theory about the formation of the Moon holds that a large body-- perhaps another planet-- collided with the young Earth. The debris from that collision presumably eventually formed the Moon. As the STEREO probes approach two Lagrange points-- positions in space where the gravity of Earth and Sun balance out-- scientists plan to use the probes to search for remnants of the hypothetical planet that might have collected in those special areas.

The hypothetical planet has been given a real name-- Theia.