The possibility of complex alien life elsewhere in the universe could be rarer than previously thought, researchers have warned, with asteroid belts considered near-vital for evolution-spurring proving in short supply. Although the centerpiece of disaster movies such as Deep Impact and Armageddon, asteroids are also believed to act as a catalyst to life, delivering water and chemicals to planets as well as punctuating species development through periodic impacts. That's a fine balance to be made, however, researchers at the University of Colorado argue, and it demands a particular - and uncommonly found - type of asteroid belt to be present.
"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," Rebecca Martin, lead author of the study and NASA Sagan Fellow, said of the discoveries. "Our study suggests that our solar system may be rather special."
Contrary to the species-ending impacts asteroids are credited with in popular media, Martin's team concluded, a reasonably well corralled field could be essential if life is ever to emerge on nearby planets. In the case of our own solar system, between Mars and Jupiter, the positioning of the millions of rocks allow water ice to remain intact, while the gravity from Jupiter itself meant the asteroids remained separate rather than coalescing into a planet.
"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" Space Telescope Science Institute astronomer Mario Livio said. "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."
However, the number of solar systems with a giant planet sitting just outside the so-called "snow line," is low. Of the 520 such planets outside of our solar system that Livio and Martin examined, only 19 are found in the correct position,
That doesn't mean life outside of Earth is impossible, the researchers argue, but it does give future research a place to start looking. "Based on our scenario," Livio concluded, "we should concentrate our efforts to look for complex life in systems that have a giant planet outside of the snow line."