What are the implications of a star systems missing a massive gas giant such as our Solar Systems'Jupiter --it
could imply conditions of massive bombardment from comets and asteroids
that would prevent the development of advanced life.
The artist's impression below of the debris disc and
planets around the star known as Gliese 581, superimposed on Herschel
PACS images at 70, 100 and 160 micrometre wavelengths. The line drawing
superimposed on the Herschel image gives a schematic representation of
the location and orientation of the star, planets and disc, albeit not
to scale. GJ 581’s planets have masses between 2 and 15 Earth masses and are all located within 0.22 Astronomical Units (AU,
where 1 AU is the distance between Earth and our Sun) of the central
star. Minus a Jupiter-like gas giant, a vast debris disc extends from
approximately 25 AU to 60 AU.
Two NASA astronomers
recently suggested 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.
Rebecca Martin, a NASA Sagan Fellow from the University of Colorado in Boulder, and astronomer Mario Livio of the Space Telescope Science Institute have concluded that Solar systems with life-bearing planets may be rare if they are dependent on the presence of asteroid belts of just the right mass.
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."
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 illustration below 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.
New research based on an analysis of theoretical models and archival
observations, including infrared data from NASA's Spitzer Space
Telescope, suggests that the second scenario may also be important for
the development of life in other solar systems.
The Sagan Fellowship Program is administered by the NASA Exoplanet Science Institute at theCalifornia Institute of Technology in Pasadena, Calif., whose purpose is to advance the scientific and technical goals of NASA's Exoplanet Exploration Program.
Image credits: ESA/AOES and Hubble/IRTF Composite Image of Jupiter
Storms. Credit: NASA, ESA, IRTF, A. Sánchez-Lavega and R. Hueso
(Universidad del País Vasco, Spain)
Source: The Daily Galaxy via http://planetquest.jpl.nasa.gov
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