On Oct 5, 1:23*pm, Pat Flannery wrote:
Two Earth-sized planets in orbit around a double star... then, one
day...KA-BLEWEY!http://www.astrobio.net/news/index.p...ticle&sid=2886

Pat

This is interesting. Two stars come to orbit one another very
closely. So, closely that astronomers thought it was one star. They
wondered about the excessive dust. Two planetary systems orbiting in
opposite directions in nearly the same plane. This would be like
driving down the wrong side of the highway. Its only a matter of time
before a collision! Then of course, debris flies off in all
directions, showering the other planets. Eventually, the angular
momentum of both systems cancel, and it all falls down!

Sheez.

At the end they note that even small collisions have dire consequences
- yeah.

Here's what would happen if a small Ceres sized object struck Earth

http://www.youtube.com/watch?v=yYgEwXWilUc

Of course, the moon was formed when the proto Earth - smaller than
today's Earth - collided with a mars sized object.- moving in the same
direction. Moving in opposite directions, both planets would have
been obliterated.

Moving in the same direction, two planets would have hit one another
with a 10 km/sec speed, which would be less than their escape velocity
- leaving debris that would eventually fall back together - leaving
perhaps a large moon. Off axis collisions would cause the objects to
spin wildly as the careened off one another.

Moving in opposite directions in the same plane the two planets would
hit one another with 70 km/sec speed, ejecting material across the
ecliptic.- off axix collisions would have caused objects to spin even
more wildly - tearing apart from the spin.