Asteroid Belts at Just the Right Place are Friendly to Life

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Whitney Clavin 818-354-4673
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Cheryl S. Gundy 410-338-4707
Space Telescope Science Institute, Baltimore, Md.
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News release: 2012-345 Nov. 1, 2012

Asteroid Belts at Just the Right Place are Friendly to Life

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.php?release=2012-345&cid=release_2012-345

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 Caltech.

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

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