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Subject: Astronomers Find Life On Earth
http://cfa-www.harvard.edu/press/pr0223.html
Harvard-Smithsonian Center for Astrophysics
Release No.: 02-23
For Release: October 30, 2002
ASTRONOMERS FIND LIFE ON EARTH
Cambridge, MA - Now that the discovery of extrasolar planets, or planets
around distant stars, has become relatively routine, scientists are now
tackling the next step: finding life-bearing worlds. To do this, observers
must know what signs to look for in the feeble light from these faraway
planets.
Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA), in
collaboration with researchers at the University of Arizona's Steward
Observatory, have identified key signatures of life by studying
Earthshine-the light of the Earth reflected off the dark side of the Moon.
They found clear signs of water and an oxygen atmosphere, as well as
tentative signs of plant life. Their findings give a clear indication of
what "fingerprints" to search for when seeking life on Earth-like worlds
orbiting distant stars.
"Our research is paving the way for future missions like the Terrestrial
Planet Finder," says Smithsonian astronomer Wes Traub. "Hopefully, within
the next 10 years astronomers will be able to confidently say that some
as-yet-undiscovered planet is a living world like our own."
Archetypal Earth
So far, astronomers can only detect Jupiter-like planets around other
stars because such planets are large and create strong gravitational
signals. However, as technology continues to improve, astronomers soon
will be able to locate Earth-like extrasolar planets and study their dim
light to search for signs of life. To know what to look for, they must use
the example of the one planet where life is known to exist: the Earth.
To replicate the view that a distant astronomer would have if studying the
Earth from another planet, Traub and his colleagues used the nearby Moon
as a mirror. Using the Steward Observatory 90-inch telescope at Kitt Peak,
Arizona, they measured both the light of Earthshine from the Moon and the
light of the Moon itself, then corrected the Earthshine to determine how
the Earth would appear to a faraway observer. They compared this measured
spectrum to a model created by Traub and CfA's Ken Jucks.
The team found that Earthlight shows strong evidence for water - a
necessary ingredient for life as we know it - and for molecular oxygen,
which must be continually replenished by the processes of life to remain
in the atmosphere. They also found features that suggested the presence of
chlorophyll, indicating the existence of land plants.
The latter showed up as bright reflections in the far-red region of the
visible spectrum. This "red edge" is a well-known signature of
chlorophyll, which appears green to us only because our eyes aren't very
sensitive at the red end of the visible spectrum.
The team also suggests that changes for finding life-bearing worlds are
improved because the signatures can develop early in a planet's history
and last for a long time. Our home planet has maintained an oxygen
atmosphere for the past two billion years, and has shown a "red edge"
since the first land plants evolved 500 million years ago.
"If someone out there is watching our solar system," Traub points out,
"they could have detected plant life here long before any intelligent life
appeared."
Findings Match Galileo
These measurements complement those made by the Galileo spacecraft during
a 1990 fly-by of Earth. As reported in the October 21, 1993 issue of
Nature, instruments aboard the spacecraft also found evidence of gaseous
oxygen and land plants.
However, the Galileo measurements were made while it was close to the
Earth and show conditions only in limited areas of the planet's surface.
Studying Earthlight, on the other hand, yields a spectrum integrated over
the entire visible surface of the planet, which matches the view that
would be available to a distant astronomer in another star system.
The measurements by Traub and his colleagues, reported in the July 20,
2002 issue of The Astrophysical Journal, were taken over two nights. The
astronomers suggest that follow-up studies be conducted over a longer
period of time to see how Earthlight changes as different areas of the
planet rotate into view, and as cloud cover changes.
NOTE TO EDITORS: An artist's depiction of our world when the Earthshine
measurements were taken, enhanced to show the "red edge" from vegetation,
is online at http://cfa-www.harvard.edu/ep/pressrel/pr0223_image.html
Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center
for Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists organized into seven research divisions study the origin,
evolution, and ultimate fate of the universe.
For more information, contact: David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
[EMAIL PROTECTED]
Christine Lafon
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
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