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 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.
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 of extrasolar
Jupiter-sized planets and debris disks around young stars. "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.
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This illustration shows three possible scenarios for the evolution of asteroid belts. In the top panel, a Jupiter-size planet migrates through the asteroid belt, scattering material and inhibiting the formation of life on planets. The second scenario shows our solar-system model: a Jupiter-size planet that moves slightly inward but is just outside the asteroid belt. In the third illustration, a large planet does not migrate at all, creating a massive asteroid belt. Material from the hefty asteroid belt would bombard planets, possibly preventing life from evolving. (Credit: NASA/ESA/A. Feild, STScI)
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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 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."
In fact, during the solar system's infancy,
the asteroid belt probably had enough material to make another Earth, but
Jupiter's presence and its small migration towards the sun caused some of the
material to scatter. Today, the asteroid belt contains less than one percent of
its original mass. 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 protoplanetary 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 NASA’s 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, 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.
The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space Agency. NASA's
Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Md., conducts Hubble science
operations. STScI is operated by the Association of Universities for Research
in Astronomy, Inc., in Washington, D.C.
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