This artist's concept shows Kepler-186f, the first roughly Earth-size planet found to be orbiting in the habitable zone of its star. Searches for signs of life on worlds such as Kepler-186f could focus on water, oxygen and alien versions of chlorophyll, researchers say.

The next generation of space telescopes hunting for signs of extraterrestrial life should focus on water, then oxygen and then alien versions of the plant chemical chlorophyll, a new study suggests.

In the past 20 years or so, astronomers have confirmed the existence of nearly 2,000 worlds outside Earth's solar system. Many of these exoplanets lie in the habitable zones of stars, areas potentially warm enough for the worlds to harbor liquid water on their surfaces. Astrobiologists hope that life may someday be spotted on such alien planets, since there is life pretty much everywhere water exists on Earth.

One strategy to discover signs of such alien life involves looking for ways that organisms might change a world's appearance. For example, chemicals typically shape what are known as the spectra seen from planets by adding or removing wavelengths of light. Alien-hunting telescopes could look for spectra that reveal chemicals associated with life. In other words, these searches would focus on biosignatures — chemicals or combinations of chemicals that life could produce, but that processes other than life could not or would be unlikely to create. [5 Bold Claims of Alien Life]

The scientists found that water would be the easiest to detect.

"Water is a very common molecule, and I think a mission to take spectra of exoplanets should certainly look for water," said Brandt, the lead study author. "Indeed, we have found water in a few gas giants more massive than Jupiter orbiting other stars."

In comparison, oxygen is more difficult to detect than previously thought, requiring scientific instruments approximately twice as sensitive as those needed to detect water and significantly better at discriminating between similar colors of light.

"Oxygen, however, has only been a large part of Earth's atmosphere for a few hundred million years," Brandt said. "If we see it in an exoplanet, it probably points to life, but not finding oxygen certainly does not mean that the planet is sterile."

Although a well-designed space telescope could detect water and oxygen on a nearby Earth twin, the astrophysicists found the instrument would need to be significantly more sensitive, or very lucky, to see chlorophyll. Identifying this chemical typically requires scientific instruments about six times more sensitive than those needed for oxygen. Chlorophyll becomes as detectable as oxygen only when an exoplanet has a lot of vegetation and/or little in the way of cloud cover, researchers said.

Chlorophyll slightly reddens the light from Earth. If extraterrestrial life does convert sunlight to energy as plants do, scientists expect that the alien process might use a different pigment than chlorophyll. But alien photosynthesis could also slightly redden planets, just as chlorophyll does.

"Light comes in packets called photons, and only photons with at least a certain amount of energy are useful for photosynthesis," Brandt said. Chlorophyll reflects photons that are too red and low in energy to be used for photosynthesis, and it may be reasonable to assume that extraterrestrial pigments would do the same thing, Brandt noted.

The researchers suggest a strategy for discovering Earthlike alien life that first looks for water, then oxygen on the more favorable planets and finally chlorophyll on only the most exceptionally promising worlds.

"The goal of a future space telescope will be primarily to detect water and oxygen on a planet around a nearby star," Brandt said. "The construction and launch of such a telescope will probably cost at least $10 billion and won't happen for at least 20 years — a lot of technology development needs to happen first — but it could be the most exciting mission of my lifetime."

Brandt and Spiegel detailed their findings online Sept. 1 in the journal Proceedings of the National Academy of Sciences.

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