Looking for a long-lost solar system sibling

2009-04-17 00:00

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 rumoured 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 Centre 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.

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.

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