Wednesday, June 10, 2009
As astronomers gaze toward nearby planetary systems in search of
life, they are focusing their attention on each system's habitable
zone, where heat radiated from the star is just right to keep a
planet's water in liquid form.
A number of planets have been discovered orbiting red dwarf stars,
which make up about three-quarters of the stars close to our solar
system. Potentially habitable planets must orbit close to those
stars – perhaps one-fiftieth the distance of Earth to the sun –
since those stars are smaller and generate less heat than our
sun.
But new calculations indicate that, with planets so close, tidal
forces exerted on planets by the parent star's gravity could limit
what is regarded as a star's habitable zone and change the criteria
for planets where life could potentially take root.
Scientists believe liquid water is essential for life. But a planet
also must have plate tectonics to pull excess carbon from its
atmosphere and confine it in rocks to prevent runaway greenhouse
warming. Tectonics, or the movement of the plates that make up a
planet's surface, typically is driven by radioactive decay in the
planet's core, but a star's gravity can cause tides in the planet,
which creates more energy to drive plate tectonics.
"If you have plate tectonics, then you can have long-term climate
stability, which we think is a prerequisite for life," said Rory
Barnes, a University of Washington postdoctoral researcher in
astronomy.
However, tectonic forces cannot be so severe that geologic events
quickly repave a planet's surface and destroy life that might have
gotten a foothold, he said. The planet must be at a distance where
tugging from the star's gravitational field generates tectonics
without setting off extreme volcanic activity that resurfaces the
planet in too short a time for life to prosper.
Barnes is lead author of a paper to be published by The
Astrophysical Journal Letters that uses new calculations from
computer modeling to define a "tidal habitable zone." Co-authors
are Brian Jackson and Richard Greenberg from the University of
Arizona and Sean Raymond from the University of Colorado. The
research was funded by NASA.
"Overall, the effect of this work is to reduce the number of
habitable environments in the universe, or at least what we have
thought of as habitable environments," Barnes said. "The best
places to look for habitability are where this new definition and
the old definition overlap."
The new calculations have implications for planets previously
considered too small for habitability. An example is Mars, which
used to experience tectonics but that activity ceased as heat from
the planet's decaying inner core dissipated.
But as planets get closer to their suns, the gravitational pull
gets stronger, tidal forces increase and more energy is released.
If Mars were to move closer to the sun, the sun's tidal tugs could
possibly restart the tectonics, releasing gases from the core to
provide more atmosphere. If Mars harbors liquid water, at that
point it could be habitable for life as we know it.
Various moons of Jupiter have long been considered as potentially
harboring life. But one of them, Io, has so much volcanic activity,
the result of tidal forces from Jupiter, that it is not regarded as
a good candidate. Tectonic activity remakes Io's surface in less
than 1 million years.
"If that were to happen on Earth, it would be hard to imagine how
life would develop," Barnes said.
A potential Earth-like planet, but eight times more massive, called
Gliese 581d was discovered in 2007 about 20 light years away in the
constellation Libra. At first it was thought the planet was too far
from its sun, Gliese 581, to have liquid water, but recent
observations have determined the orbit is within the habitable zone
for liquid water. However, the planet is outside the habitable zone
for its sun's tidal forces, which the authors believe drastically
limits the possibility of life.
"Our model predicts that tides may contribute only one-quarter of
the heating required to make the planet habitable, so a lot of heat
from decay of radioactive isotopes may be required to make up the
difference," Jackson said.
Barnes added, "The bottom line is that tidal forcing is an
important factor that we are going to have to consider when looking
for habitable planets."
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University of Washington
Thanks to University of Washington for this article.