Asteroid belts might be the key to intelligent life evolving in other solar systems - with impacts delivering water to Earth-like worlds and spurring evolution.
The finding could help scientists pinpoint alien worlds which might harbour life.
Out of 520 systems with large planets scanned by Nasa's Spitzer space telescope, 19 were found to have asteroid belts in the 'perfect' position to spur life.
The scientists suggest that these should be the focus of our search for extraterrestrial life.
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Asteroids are often considered a threat to life due to their potential to impact Earth and trigger mass extinctions.
But the rocks are thought to have delivered water to the young Earth, and to have spurred the evolution of complex mammals by wiping out the disaster.
The new research by Rebecca Martin, a NASA Sagan Fellow from the University of Colorado in Boulder, and astronomer Mario Livio of the Space Telescope Science Institute in Baltimore, Md.
suggests that asteroid collisions with planets may provide a boost to the birth and evolution of complex life.
They suggest that the size and location of an asteroid belt, shaped by the evolution of the sun's protoplanetary disk and by the gravitational influence of a nearby giant Jupiter-like planet, may determine whether complex life will evolve on an Earth-like planet.
Asteroids may have delivered water and organic compounds to the early Earth.
Occasional asteroid impacts might accelerate the rate of biological evolution by disrupting a planet's environment to the point where species must try new adaptation strategies.
"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," said Martin, the study's lead author. "Our study suggests that our solar system may be rather special."
The asteroid belt in our solar system, located between Mars and Jupiter, is a region of millions of space rocks that sits near the "snow line," which marks the border of a cold region where volatile material such as water ice are far enough from the sun to remain intact.
At the time when the giant planets in our solar system were forming, the region just beyond the snow line contained a dense mix of ices, rock and metals that provided enough material to build giant planets like Jupiter.
When Jupiter formed just beyond the snow line, its powerful gravity prevented nearby material inside its orbit from coalescing and building planets. Instead, Jupiter's influence caused the material to collide and break apart. These fragmented rocks settled into an asteroid belt around the sun.
"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," Livio explained. "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."
The duo then studied observations of the 520 giant planets found outside our solar system.
Only 19 of them reside outside the snow line, suggesting that most of the giant planets that may have formed outside the snow line have migrated too far inward to preserve the kind of slightly-dispersed asteroid belt needed to foster enhanced evolution of life on an Earth-like planet near the belt. Apparently, less than four percent of the observed systems may actually harbor such a compact asteroid belt.
"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," Livio said.