Alien "ID Chart" to Aid Search for Extraterrestrial Life
Brian Handwerk
for National Geographic News

The search for planets with extraterrestrial life has gotten a new tool:
an "ID chart" that scientists will use to compare alien worlds with
Earth as it has appeared over the eons.
Many astronomers say they expect to find Earthlike planets soon, when
better technology enables them to spot small, distant worlds.

"We believe that within 10 or 15 years we'll find the first planet
that's Earthlike," said Lisa Kaltenegger, an astrophysicist with the
Harvard-Smithsonian Center for Astrophysics (CfA).
"Then the question will be, Is this a habitable planet?"

To answer that question Kaltenegger and colleagues have created a
historical model of the only planet known to have intelligent
life—Earth.
"We created a fingerprint for the Earth throughout its evolution so that
we can compare it to any planet that we find and, we think, say if there
is life on it," Kaltenegger explained.
(CfA scientists also announced today the discovery of the largest planet
ever found.)

"Fingerprint" of Extraterrestrial Life
The gases in a planet's atmosphere provide the unique fingerprint for
such a comparison.
Scientists examine atmospheric conditions on worlds too distant to visit
by using visible and infrared light.
The mix of gases in a planet's atmosphere creates a unique spectrum—a
colorful fingerprint—that reveals conditions on the planet's surface.

"It's interesting that the only way we can tell what's going on in
oceans or on the ground is by measuring waste gases in the atmosphere,"
said Wesley Traub of NASA's Jet Propulsion Laboratory in Pasadena,
California.
The team's members mapped the atmospheric changes that occurred during
Earth's 4.5 billion years of evolution.

Virtual Solar System
They identified six different epochs—stages in the evolution of
carbon-based life—so that they can compare the spectra of those
atmospheres with those of any promising new planets.
"If an extrasolar planet is found with a spectrum similar to one of our
models, we potentially could characterize that planet's geological
state, its habitability, and the degree to which life has evolved on
it," Traub said.

For example, the presence of very early life could be indicated by
rising methane levels like those created when anaerobic bacteria, which
grow in the absence of oxygen, first appeared on Earth some 3.5 billion
years ago.
More advanced life could also be detected.

"Two good signs of life would be large amounts of oxygen [which made
multicellular life possible on Earth] and nitric oxide, which is
produced by life and only by life," Traub said.