Astronomers Find Life on Earth

Image: An artist’s depiction of our world when the Earthshine measurements were taken, enhanced to show the “red edge” from vegetation. A distant astronomer on another planet would see the spectral signatures of our oxygen atmosphere, water, and chlorophyll from land plants. [enlarge] Credit: John Walker’s “Earth Viewer,” Christine Lafon, (Harvard-Smithsonian Center for Astrophysics).

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.