A Serious Search for a Lost Planet

"The Search for the Solar System's Lost Planet"
Space.com (April 13, 2009)

"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,'..."

The idea that something about the size of Mars collided with Earth early on explains quite a few things about the moon. Quite a lot of the debris thrown up would have gone in orbit around Earth, and fallen together to form the moon. Extreme conditions from the collision would explain quite a lot about the moon as it exists now.

As for the hypothetical planet: it may have formed in Earth's L4 or L5 Lagrange point, been nudged out of the stable position by growth of other planets, eventually hitting Earth. There wouldn't be much left now - but STEREO spacecraft's mission takes them to the L4 and L5 Lagrange points - which gives an opportunity to look for leftovers. The STEREO space craft were designed as solar observatories - but can be programmed for other tasks.

Interesting times, these that we live in.