November 1, 2012
Asteroid Belt Needed For Life-Bearing Planets
Lee Rannals for redOrbit.com - Your Universe Online
The scientists say that the asteroid belt, shaped by the evolution of the Sun's planet-forming disk and by the gravitational influence of a nearby giant Jupiter-like planet, determines whether complex life evolves on an Earth-like planet.
Asteroids have delivered water and organic compounds to the early Earth, and the theory of punctuated equilibrium states that occasional asteroid impacts might accelerate the rate of biological evolution by disrupting a planet's environment.
Scientists performing the latest study based their conclusion on an analysis of theoretical models and archival observations, including infrared data from NASA's Spitzer Space Telescope.
"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, the study's lead author. "Our study suggests that our solar system may be rather special." Martin is a NASA Sagan Fellow from the University of Colorado in Boulder.
The astronomers suggest that the location of an asteroid belt relative to a Jupiter-like planet is not an accident.
The asteroid belt 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 like water ice is far enough from the sun to remain intact.
When Jupiter formed just beyond the snow line, its powerful gravity prevented nearby material inside its orbit from coalescing and building planets.
"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 astronomer 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."
The team proposed that asteroid belts in other solar systems would always be located about at the snow line. In order to test their hypothesis, they created models of planet-forming disks around young stars, and calculated the location of the snow line in those disks based on the mass of the central star.
They looked at all the existing space-based infrared observations from the Spitzer Space Telescope of 90 stars having warm dust, indicating the presence of an asteroid belt-like structure. The temperature of the warm dust was consistent with that of the snow line.
"The warm dust falls right onto our calculated snow lines, so the observations are consistent with our predictions," Martin said.
The astronomers then studied observations of the 520 giant planets found outside the solar system. Only 19 of the planets reside outside the snow line. This suggests that most of the giant planets that have formed outside the snow line have migrated too far inward to preserve the kind of slightly dispersed asteroid belt needed to foster life.
Less than four percent of the observed systems may actually harbor such a compact asteroid belt, according to the research.
"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 concluded.
Image 2 (below): This illustration shows our solar-system model: a Jupiter-size planet moves slightly inward but is just outside the asteroid belt. Image credit: NASA/ESA/STScI