[meteorite-list] Asteroid Belts at Just the Right Place are Friendly to Life

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 1 Nov 2012 11:29:01 -0700 (PDT)
Message-ID: <201211011829.qA1IT1QJ000111_at_zagami.jpl.nasa.gov>

Asteroid Belts at Just the Right Place are Friendly to Life
Jet Propulsion Laboratory
November 01, 2012

PASADENA, Calif. -- 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.

The Sagan Fellowship Program is administered by the NASA Exoplanet
Science Institute at the California Institute of Technology in Pasadena,
Calif., whose purpose is to advance the scientific and technical goals
of NASA's Exoplanet Exploration Program. The Exoplanet Exploration
Program is managed for NASA by NASA's Jet Propulsion Laboratory in
Pasadena, Calif. Caltech manages JPL for NASA.

More information about exoplanets and NASA's planet-finding program is
at http://planetquest.jpl.nasa.gov .

JPL manages the Spitzer Space Telescope mission for NASA's Science
Mission Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at Caltech. Data are archived at the Infrared
Science Archive housed at the Infrared Processing and Analysis Center at

For more information about Spitzer, visit http://spitzer.caltech.edu and
http://www.nasa.gov/spitzer .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin at jpl.nasa.gov

Cheryl S. Gundy 410-338-4707
Space Telescope Science Institute, Baltimore, Md.
gundy at stsci.edu

Received on Thu 01 Nov 2012 02:29:01 PM PDT

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