The size and location of an asteroid belt could be a significant factor in determining whether complex life will evolve on a nearby Earth-like planet.
According to astronomers, asteroid collisions with planets may provide a boost to the birth and evolution of complex life by delivering water and organic compounds, as is likely the case with the early Earth.
And, according to the theory of punctuated equilibrium, occasional asteroid impacts might accelerate evolution by causing such disruption that species are forced to resort to new adaptation strategies.
The team based their conclusion on an analysis of theoretical models and archival observations of extrasolar Jupiter-sized planets and debris disks around young stars.
"Our study shows that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size, offering the potential for life on a nearby rocky planet," says Rebecca Martin of the University of Colorado in Boulder. "Our study suggests that our solar system may be rather special."
Our own asteroid belt, located between Mars and Jupiter, sits near the 'snow line', beyond which volatile material such as water ice is far enough from the Sun to remain intact. When the giant planets in our solar system were forming, the region just beyond the snow line contained enough material to build giant planets like Jupiter.
And when Jupiter formed just beyond the snow line, its powerful gravity prevented nearby material inside its orbit from coalescing and building planets. Instead, its influence caused the material to collide and break apart, forming the asteroid belt.
"To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt and that migrated a little bit, but not through the belt," says Mario Livio of the Space Telescope Science Institute.
"If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve."
To test their proposal, Martin and Livio created models of protoplanetary disks around young stars and calculated the location of the snow line in those disks based on the mass of the central star.
They then looked at all the existing space-based infrared observations from NASA's Spitzer Space Telescope of 90 stars having warm dust, which could indicate the presence of an asteroid belt-like structure.
"The warm dust falls right onto our calculated snow lines, so the observations are consistent with our predictions," says Martin.
"Based on our scenario, we should concentrate our efforts to look for complex life in systems that have a giant planet outside of the snow line."