While NASA’s James Webb Space Telescope — which is scheduled to launch in 2018 — will be capable of finding signs of life on nearby exoplanets, a broad and bona fide hunt for life beyond Earth’s neighborhood will require bigger spacecraft that are not even on the agency’s books yet, experts say.

“To find evidence of actual life is going to take another generation of telescopes,” JWST telescope scientist Matt Mountain, director of the Space Telescope Science Institute in Baltimore, said July 14 during a NASA briefing. “And to do that, we’re going to need new rockets, new approaches to getting into space, new approaches to large telescopes — highly advanced optical systems.” 

The $8.8 billion JWST features 18 hexagonal mirror segments that will work together to form one 6.5-meter mirror — larger than any other mirror that’s ever flown in space, NASA officials said. For comparison, the Hubble Space Telescope sports a 2.4-meter primary mirror.

JWST is optimized to view in infrared light. The telescope should be able to do lots of different things during its operational life, researchers say, including scanning the atmospheres of alien planets for oxygen and other gases that could be produced by living organisms. 

JWST will work in concert with another NASA space mission in this regard, performing follow-up observations on promising nearby worlds found by the agency’s Transiting Exoplanet Survey Satellite (TESS), which is scheduled to blast off in 2017.

“With the James Webb, we have our first chance — our first capability of finding signs of life on another planet,” Massachusetts Institute of Technology astrophysicist Sara Seager said during the briefing. “Now nature just has to provide for us.”

But nature may not be so willing, at least during the JWST mission, Seager and other experts stressed. And it all comes down to numbers.

The Milky Way galaxy teems with at least 100 billion planets, 10 to 20 percent of which, Mountain said, likely circle in their host star’s “habitable zone” — that just-right range of distances that could allow liquid water to exist on a world’s surface. If there is nothing terribly special about Earth, then life should be common throughout the cosmos, many scientists think.

But most exoplanets are very far away, and all of them are faint. JWST, while large by current standards, will not have enough light-collecting area to investigate more than a handful of potentially habitable planets, researchers say.

A spacecraft with a 10-meter mirror would give researchers a much better chance of finding biosignatures in alien atmospheres, but Mountain would like something even bigger. “With a 20-meter telescope, we can see hundreds of Earth-like planets around other stars,” he said. “That’s what it takes to find life.”

There are no concrete plans to build and launch such a large space telescope, whose size would pose a number of logistical and engineering challenges. However, JWST is a potentially big step along the way to this goal.

For example, the JWST team figured out how to make mirror segments with incredible precision — a skill that could come in handy down the road.

“They’re basically perfect,” said JWST senior project scientist John Mather of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who won a Nobel Prize in 2006 for his work with the agency’s Cosmic Background Explorer satellite. “If we were to expand the mirror to the size of the continental United States, the mirror would be accurate to within 3 inches [about 7.5 centimeters],” Mather said. “This is completely amazing technology we have now mastered and are using.”