Alien "ID Chart" to Aid Search for Extraterrestrial Life

Brian Handwerk
for National Geographic News
September 14, 2006
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.

(See National Geographic magazine's "Search for Other Earths.")

"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.

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.

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