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."
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.
advertisement
