The vast majority of planets discovered outside our own solar system have been "gas giants" like Jupiter. Besides being far larger than Jupiter, however, they also tend to have orbits quite close to their parent stars - often closer than Mercury (77 million km).

According to new computer models, this does not necessarily rule out the existence of Earth-like planets in these solar systems. These models suggest that gas giants form farther out from their parent stars, then migrate inward. Initially, this migration throws matter that would form small, rocky "terrestrial" planets into highly eccentric and unstable orbits.

Eventually, however, the gas giants form a sort of gravitational "cushion," allowing terrestrial-type planets to settle into stable orbits in the "Goldilocks zone," where water can exist in a liquid state, and thus allowing for the possibilty of the existence of carbon-based life as we know it.