Our solar system has a pleasing architecture. There are four inner rocky, or "terrestrial", planets on tight, closely spaced orbits. Then comes the asteroid belt. After that comes four outer gas/ice giants on much more widely space orbits.
It's quite lovely but may also be quite unusual.
Ever since we began discovering exoplanets orbiting other stars the question "Are we weird?" has been paramount in the minds of astronomers. It's not just a matter of aesthetics. Having such a clean separation between rocky planets and giants may be essential to habitability, allowing the rocky worlds time and space to develop stable climates etc. for the development of life.
Now new results from the University of Maryland and the Tokyo Institute of Tecnnology indicate that our solar system may, in fact, be quite unusual. Using computer simulations the group finds that young Earth-like planets may have hard time in the presence of young, unstable giant planets. From the abstract:
"Our results indicate that, unless the dynamical instability among giant planets is either absent or quiet like planet-planet collisions, most test particles within the orbits of giant planets at a few AU may be gone."
"Test-particles" here means the terrestrial planets (yikes!). Thus, unless the giant planets are very stable in their orbits (which is unlikely early on) then most small worlds get booted out from the kinds of orbits we find in our system.
Gone, baby gone.