The Search for the Solar System's Lost Planet

The solar system might once have had another planet named Theia, which may have helped create our own planet's moon.

Now two spacecraft are heading out to search for leftovers from this rumored sibling, which would have been destroyed when the solar system was still young.

"It's a hypothetical world. We've never actually seen it, but some researchers believe it existed 4.5 billion years ago ? and that it collided with Earth to form the moon," said Mike Kaiser, a NASA scientist at the Goddard Space Flight Center in Maryland.

Theia is thought to have been about Mars-sized. If the planet crashed into Earth long ago, debris from the collision could have clumped together to form the moon. This scenario was first conceived by Princeton scientists Edward Belbruno and Richard Gott.

Many researchers now figure that indeed some large object crashed into Earth, and the resulting debris coalesced to form the moon. It is unclear though if that colliding object was a planet, asteroid or comet.

In any case, the debris that would have spun out from the two slamming bodies would have mixed together, and could explain some aspects of the moon's geology, such as the size of the moon's core and the density and composition of moon rocks.

Scientists are hoping NASA's twin STEREO probes, launched in 2006, will be able to discover leftover traces of Theia that may finally help close the case on the birth of our moon.

So far, signs of Theia have proved elusive to telescopes searching from Earth. But the STEREO spacecraft are set to enter special points in space, called Lagrangian points, where the gravity from the Earth and the sun combine to form wells that tend to collect solar system detritus. [Click here for an animation that explains Lagrangian points.]

"The STEREO probes are entering these regions of space now," Kaiser, a STEREO project scientist, said. "This puts us in a good position to search for Theia's asteroid-sized leftovers."

By visiting the Lagrangian points directly, STEREO will be able to hunt for Theia chunks up close. The nearest approach to the bottoms of the gravitational wells will come in September and October 2009.

"STEREO is a solar observatory," Kaiser said. "The two probes are flanking the sun on opposite sides to gain a 3-D view of solar activity. We just happen to be passing through the L4 and L5 Lagrange points en route. This is purely bonus science."

Scientists think Theia may even have formed in one of these gravitational points of balance from the accumulation of flotsam that had built up there.

"Computer models show that Theia could have grown large enough to produce the moon if it formed in the L4 or L5 [Lagrangian] regions, where the balance of forces allowed enough material to accumulate," Kaiser said. "Later, Theia would have been nudged out of L4 or L5 by the increasing gravity of other developing planets like Venus and sent on a collision course with Earth."

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Editor's Note: This story was updated at 1:50 p.m. ET to properly credit Edward Belbruno and Richard Gott with the idea that a planet like Theia might have impacted Earth to form the moon.