Artist's conception of the hypothetical impact of Theia and young Earth. Credit: NASA/GSFC
Artist's conception of the hypothetical impact of Theia and young Earth. Credit: NASA/GSFC
updated 4/13/2009 12:26:15 PM ET 2009-04-13T16:26:15

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

Now two spacecrafts 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, called the "giant impact hypothesis," 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. However, it is unclear whether 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.

Slideshow: Month in Space: January 2014 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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Photos: All-time top 10 astronomy pictures

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  1. Picking the top ten of astronomy

    Every day since June 16, 1995, Professor Robert Nemiroff at Michigan Technological University and NASA scientist Jerry Bonnell have posted an image on the Web as their Astronomy Picture of the Day. Click through 10 of Nemiroff's favorites in this slideshow, which pays tribute to an event called 100 Hours of Astronomy. Astronomers around the world are encouraging people to peer at the skies just as Galileo did 400 years ago – and just as the stargazers in this image are doing. (Babak Tafreshi / TWAN) Back to slideshow navigation
  2. The earth also rises

    All over the world, people gaze out at the horizon as the moon rises – and perhaps ponder distant worlds. This famous shot, made as Apollo 8 astronauts rounded the far side of the moon in 1968, pulls a switcheroo. "I think maybe people put their existence in some kind of context," Nemiroff says of the image's impact. "We are all on the same big blue marble." (NASA) Back to slideshow navigation
  3. Man on the moon

    Astronaut Harrison Schmitt walks alongside a moon buggy during the final lunar landing mission of the Apollo program in December 1972. While there are many historic images from the surface of the moon, Nemiroff says, "I like this one. It shows the moon surface, it shows some desolation, it shows the magnificence. And there's a human for scale, so people can identify." Schmitt and Apollo 17 commander Gene Cernan, who took this picture, were the last astronauts to walk on the moon. (NASA) Back to slideshow navigation
  4. Sun with a monstrous claw

    Ah, the sun ... or should that be "AHHH, the sun!!" Nemiroff says our star is the most well-known object in the sky, but this image casts it in a different – and slightly terrifying – light. The view from the orbiting Solar and Heliospheric Observatory, or SOHO, shows the sun as a complex, busy sphere. On the lower left is a large flare reaching out like a giant claw. "The earth could easily fit in that claw," Nemiroff says. (NASA/ESA) Back to slideshow navigation
  5. A solar eclipse, from space

    Some people travel thousands of miles to see a total eclipse of the sun – when the full moon passes between the sun and Earth, temporarily blotting the sun from the sky. Travelers on the Mir space station captured this rare image of the spectacle as seen from space on Aug. 11, 1999. For Nemiroff, it's a fantastic educational tool: "People suddenly understand that when the moon covers the sun, there's a shadow on the earth – and if you stand in the shadow, that's where you see the total eclipse of the sun," he says. (CNES) Back to slideshow navigation
  6. Stars are born

    This iconic image of the pillars of creation in the Eagle Nebula was captured by the Hubble Space Telescope in 1995. The giant pillars are light-years-long columns of dense gas and dust that condense to form stars. "You can actually look into the pillars and see things," Nemiroff says. "The end of the topmost pillar is being boiled away by stars." The nebula is about 7,000 light-years from earth. (NASA, ESA, STScI, J. Hester and P. Scowen (ASU)) Back to slideshow navigation
  7. Perspective on the pillars

    An entirely different take on the famed pillars of creation was gained with this wide-field image of the Eagle Nebula made with the 0.9-meter telescope on Arizona's Kitt Peak. "It puts the other image that is well-known – that is in the very center – puts it in a bigger perspective," Nemiroff says. "And you can see a star nebula as an open cluster of stars forming there – the pink." (A. Rector and B.A. Wolpa (NRAO / AUI / AURA / NSF)) Back to slideshow navigation
  8. Our galactic neighbor

    The Andromeda Galaxy is our Milky Way's closest spiral galactic neighbor, located about 2 million light-years away. Though visible as a faint smudge with the naked eye, the galaxy comes to life when viewed with basic observing equipment. Amateur astronomer Robert Gendler combined 40 gray-scale frames of the galaxy to create this beautiful image. Color came from previous images. Access to these tools, Nemiroff notes, allows amateurs to contribute scientifically valuable images to astronomy. (Robert Gendler) Back to slideshow navigation
  9. Earth (and our lights) at night

    Images gathered by a swarm of Defense Department satellites were stitched together to make this snapshot of the earth at night. "It is interesting to people because people like to find where they are [and] it shows where people are on planet Earth," Nemiroff says. Urban legend, he adds, erroneously attributes the creation of the popular image to a single picture taken from the space shuttle. The thoughtful observer will realize that night is everywhere in the image, which never happens. (NASA via Getty Images file) Back to slideshow navigation
  10. The first shuttle

    On April 12, 1981, the space shuttle Columbia blasted off from NASA's Kennedy Space Center in Florida, launching a new era of human spaceflight. This photo was taken on the night before the historic launch. Nemiroff says it's a "historically fascinating, visually interesting image." The two-day checkout flight ended with a safe, airplane-style landing at Edwards Air Force Base in California. The shuttle program continues, primarily helping NASA complete the international space station. (NASA) Back to slideshow navigation
  11. Robot attacks the space station

    Nemiroff and APOD colleague Jerry Bonnell like to have fun with their Web site every now and again, and usually post something a bit tongue-in-cheek on April Fools' Day. One of Nemiroff's recent favorites shows a robot, named Dextre, apparently attacking the international space station. "You see the common earth, you see the space station, you see complex things on the space station, but then there's this strange robot thing," he says. Though truly a robot, Dextre is friendly, helping astronauts with building and repairs. (NASA) Back to slideshow navigation
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