A new study shows that planets like Earth could be rare if they depend on asteroid belts of a specific mass range. Researchers suggested that the size and location of a solar system's asteroid belt could determine whether complex life can evolve on an Earth-like planet.
"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 Rebecca Martin, study co-author and a NASA Sagan fellow at the University of Colorado. "Our study suggests that our solar system may be rather special."
The study was based on an analysis of theoretical models and archival observations, including infrared data from NASA's Spitzer telescope. It suggests that Jupiter's gravity prevented the material in our asteroid belt from forming into a planet, and that material from the asteroid belt colliding with Earth provided extinction events crucial for speeding up the evolution of life.
"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," said Mario Livio, study co-author and astronomer at 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."
Martin and Livio tested models of accretion discs around young stars to calculate the "snow line" where material such as ice can remain intact. They cross-referenced their calculations with data from NASA's Spitzer telescope, and then looked at observations of 520 giant planets found outside of our solar system. They found that only 19 of the giant planets are outside of the snow line, and that less than 4% of the observed systems may have a "compact" asteroid belt such as ours.
"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," said Livio.