For astrophysicists working to discover the origins of stars and planets, a small clue can go a long way. They can't get a close look at distant stars and planets, so they only know the barest details about other planetary systems. One such detail is that some extra-solar planets revolve around their stars in elliptical orbits rather than the nearly circular orbits that are the norm in our solar system. According to JILA Fellow Phil Armitage and graduate student Dimitri Veras, this small difference indicates a very different history - one that could have spelled doom for a planet like Earth.
Armitage and Veras believe that when a planetary system includes planets with eccentric orbits, it is a sign that the now-stable system has been through a tumultuous past. They think that such planets started out with circular orbits, but were pushed and pulled out of step, upsetting the equilibrium of the whole system. Eventually, the planets settled into their new elliptical orbits, but only after a period of instability while the system regained its balance. Using computer simulations, Armitage and Veras have investigated how this process of upset and stabilization might occur and how it might occur differently for systems with different configurations.
The chaos begins when the planets are too close together. They tug on each other with their gravity fields, pushing one another out of orbit. From here, they gain momentum, and soon they are careening toward other planets, getting them into the act. This time is a dangerous one for planets. Those close to the star have a chance of diving right in and being disintegrated. Planets far away from the star will sometimes be thrown so far that their orbit becomes "hyperbolic," which means they fly away, never to return. Occasionally planets will even smash into each other.
According to Armitage, our solar system never went through a period like this. Aside from the circular nature of our orbits, the existence of small, terrestrial planets is an argument against it. Imagine Jupiter or Saturn flying across the asteroid belt and through the neighborhood of Earth and Mars. Even without making direct contact, the gravity of a gas giant would likely destroy a smaller planet, either by sucking it in or sending it on a fatal trajectory. For this reason, Armitage and Veras think that where we find eccentric orbits, we probably won't find many Earth-like planets. Certainly, as the scientists are able to look more and more closely at neighboring stars, they will find out if their hypothesis is correct. - Julie Phillips