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."

The Theia Hypothesis

According to the giant impact hypothesis, there was once a Mars-sized body referred to as Theia orbiting in our solar system. The planet was named after the Greek Titan who gave birth to the Moon goddess Selene—a fitting name considering that the planet Theia is thought to be responsible for the birth of our moon.

Theia would have formed in about the same orbit as Earth, but about 60° ahead or behind. When the protoplanet had grown to be about the size of Mars, its size made it too heavy for its orbit to remain stable. As a result, its angular distance from Earth varied increasingly, until it finally it crashed into the Earth.

The collision would have occurred circa 4.533 billion years ago when Theia would have hit the Earth at an oblique angle, and destroyed herself in the process. Theia's mantle and a significant portion of the Earth's silicate mantle were thrust into space. The left over materials from Theia mixed with the materials from the Earth and eventually formed the Moon.

New research is validating this hypothesis, showing that the Earth’s core and the Moon’s core contain the same silicon isotopic material, which would support that the two were once a single body until a large impact separated them.

Scientists from Oxford University, University of California, and Swiss Federal Institute of Technology compared silicon isotopes from Earth rocks, as well as other materials from our solar system such as rocky materials from meteorites.

Up to about 2,900 kilometers (1,800 miles) into the Earth (not quite half way to the center), is what we know as the mantle and crust. They are predominantly formed of silicate, a compound mad of silicon, oxygen, and other elements. Past the halfway mark is a dense metallic iron material that makes up the Earth’s core.

The multinational team found that the heavier isotopes from silicate samples taken from the Earth consisted of increased amounts of the heavier isotopes of silicon. They found that Mars, the asteroid Vesta, and various chrondites (primitive meteorites that never produced ainner cores) do not contain such an arrangement, even though they have an iron core. is much smaller than the Earth (about one-eighth the size), so did not have enough mass to generate the pressure necessary to form the same core as found in the Earth.

On the other hand, the researchers found that the Moon did show a similar composition of the silicon isotopic composition as the Earth. However, it, too, is much smaller than the Earth—about one-fiftieth as large as the Earth and about one percent of the Earth’s mass—making it even less likely to have been able to generate enough pressure to form an Earth-like iron core.

However, such a core does exist at the center of the Moon, but no one can explain how it got there.

The researchers contend that the Moon indeed must have been created during a giant impact by a planet-size object (Theia) that hit during the early development of Earth. The impact was large enough that the materials, which eventually formed the Moon, mixed with the materials from the Earth, which already had a heavy silicon isotopic composition.

They state within their paper in the journal Nature: “The similar isotopic composition of the bulk silicate Earth and the Moon is consistent with the recent proposal that there was large-scale isotopic equilibration during the giant impact.”

This research is the first of its kind using isotopes in this manner and offers intriguing insights into the creation of Mars, the Earth, and the Moon. It may also help explain how life evolved on the Earth and whether or not it might have existed at some time on Mars.

Phantoms and Monsters