But most exoplanets are very far away, and all of them are faint. JWST, while large by current standards, won't have enough light-collecting area to investigate more than a handful of potentially habitable planets, researchers say.
A spacecraft with a 33-foot (10 m) mirror would give researchers a much better chance of finding biosignatures in alien atmospheres, but Mountain would like something even bigger.
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"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."
Laying the foundation 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.
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"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," Mather said. "This is completely amazing technology we have now mastered and are using."
The hunt for life on distant worlds will be a multi-generational effort that goes from TESS and JWST to other, larger space telescopes, Seager said. And overcoming the various challenges involved will almost certainly require the cooperation of a number of different countries and organizations.
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"Putting together the partnership that can find Earth 2.0 is a challenge worthy of a great generation," Mountain said.
Closer to home All of this does not necessarily mean, however, that alien life won't be detected until humanity launches an enormous space telescope. Indeed, confirmation that Earthlings aren't alone in the universe may come from worlds much closer to home.
For example, NASA's next Red Planet rover, which is due to launch in 2020, will hunt for signs of past Mars life. And both NASA and the European Space Agency have plans to mount a mission to Jupiter's ocean-harboring moon Europa, which many experts regard as the solar system's best best to host alien life.
Europe's JUpiter ICy moons Explorer (JUICE) mission is currently scheduled to blast off in 2022 to study the Jovian satellites Callisto and Ganymede in addition to Europa. NASA officials have said they hope to launch a Europa mission sometime in the mid-2020s.
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