From outer space, no other planet in our solar system resembles Earth, the blue planet. Our atmosphere, oceans and green, plant-covered continents reflect the light of the sun in a unique spectrum of colors. Earth also emits invisible infrared radiation. Should astronomers on Earth one day spy a planet in some distant star system with a similar profile, they might say the existence of life on this new world is the best explanation. 

Building upon this simple concept, astronomers Lisa Kaltenegger, Wesley Traub and Kenneth Jucks of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., have created a new guide by which astronomers can identify planets in other solar systems that may contain life. The guide is based on computer models that calculate how the reflected light and emitted radiation of the Earth appears from trillions of miles away and just how that spectrum has changed during the Earth’s geologic past.

Spectral signatures
Different atmospheric gases leave specific spectral signatures or fingerprints, both visible and infrared, on the light reflected and emitted from a planet. By collecting this light and splitting it into a rainbow spectrum of colors (as a prism does), astronomers can determine which gases are present in a planet’s atmosphere.
"By looking at a planet’s atmosphere, we can search for signs that the air has been altered by living organisms," Kaltenegger explains. "Since Earth is the only known planet with life, we studied the history of Earth’s atmosphere to learn what signs to seek on other worlds," she adds.

Although current space telescopes are unable to examine the faint light from planets in distant solar systems, future instruments will have that capability. The work of Kaltenegger, Traub and Jucks is in anticipation of that day. Many astronomers believe the search for extraterrestrial life will meet with success in the next 10 to 20 years.

To find other life-bearing worlds, astronomers plan to first look in distant solar systems for planets in Earthlike orbits around their stars. These orbits are in what astronomers call "habitable zones"--areas around a star where the temperature allows for the presence of water in a liquid state.

To get its start, life on Earth required both land and liquid water. So researchers will be on the lookout for small, rocky planets orbiting in a "habitable zone" around a star.

Methane
In creating their models, the scientists also took into consideration that Earth’s spectral profile has changed dramatically in the past 4.5 billion years. Today, the air we breathe consists of about three-fourths nitrogen and one-fourth oxygen. Four billion years ago, no oxygen was present. Instead, Earth’s atmosphere was a blend of carbon dioxide, nitrogen and hydrogen sulfide that would be toxic to humans. Yet life arose and flourished in this early atmosphere.
Earth’s first inhabitants were anaerobic bacteria--organisms that can live without oxygen. Those bacteria pumped large amounts of methane into the planet’s atmosphere, changing it significantly. If anaerobic bacteria exist on another planet, future space missions might detect a methane fingerprint in the planet’s atmosphere.

But natural processes like volcanism also can inject methane into the air, Kaltenegger cautions. "Methane itself is not an unambiguous sign of life. But detecting both methane and oxygen at the same time is an excellent biosignature."

Oxygen
About 2.5 billion years ago, drastic changes permanently shifted Earth’s atmospheric balance. Blue-green algae appeared in the oceans and began emitting large amounts of oxygen into the air. The oxygen reacted with methane, clearing away most of that gas. It also suffocated many of the anaerobic bacteria that had ruled the world until that time. Soon, blue-green algae became the new dominant life form and Earth’s atmosphere gained its first free oxygen. Two billion years ago, Earth’s oxygen revolution was fully under way.

"When aerobic bacteria displaced anaerobic bacteria as the dominant life form, they introduced oxygen to Earth’s atmosphere," says Traub, who also works at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.  "That oxygen made multicellular life, including human life, possible.

Complex, diverse
"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 continues.

On Earth, life continued to evolve from blue-green algae to more complex organisms, yielding the great diversity of species now present on this planet. Some scientists are skeptical that life forms as complex as those found on Earth exist elsewhere in the galaxy.
"The first extraterrestrial life we discover may be just slimes, molds and bacteria," Kaltenegger says.

Nevertheless, discovering life elsewhere in the universe surely would count as one of the greatest and most profound moments in human history.

"Looking up at the night sky and knowing that planets like ours, complete with life, exist out there somewhere would forever change our view of the universe," Kaltenegger says. “The sky would be even richer because we could start to investigate what those other worlds are really like and how they have evolved.”