Solar systems with life-bearing
planets may be rare if they are dependent on the presence of asteroid belts of
just the right mass, according to a study 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.
They suggest that the size and
location of an 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, may determine whether complex life will evolve on an
Earth-like planet.
This might sound surprising
because asteroids are considered a nuisance due to their potential to impact
Earth and trigger mass extinctions. But an emerging view proposes that asteroid
collisions with planets may provide a boost to the birth and evolution of
complex life.
Asteroids may have delivered
water and organic compounds to the early Earth. According to the theory of
punctuated equilibrium, 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.
The astronomers 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 Martin, the
study's lead author. "Our study suggests that our solar system may be
rather special."
The findings will appear today in
the Monthly Notices of the Royal Astronomical Society: Letters.
Martin and Livio suggest that the
location of an asteroid belt relative to a Jupiter-like planet is not an
accident. 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 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.
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."
Using our solar system as a
model, Martin and Livio proposed that asteroid belts in other solar systems
would always be located approximately at the snow line. To test their proposal,
Martin and Livio 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 then looked at all the
existing space-based infrared observations from the Spitzer Space Telescope of
90 stars having warm dust, which could indicate 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 duo then studied observations
of the 520 giant planets found outside our solar system. Only 19 of them reside
outside the snow line. This suggests that most of the giant planets that may
have formed outside the snowline 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.
Source: JPL
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