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Humanity will probably have to wait a few decades to find out if life is common beyond our own solar system.
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 thorough hunt for life beyond Earth's neighborhood will require bigger spacecraft that aren't even on the agency's books yet, experts say.
"To find evidence of actual life is going to take another generation of telescopes," Webb Telescope scientist Matt Mountain, director of the Space Telescope Science Institute in Baltimore, said during a NASA briefing Monday. "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." [10 Exoplanets That Could Host Alien Life]
A chance to find signs of life
The $8.8 billion Webb Telescope features 18 hexagonal mirror segments that will work together to form one 21-foot-wide (6.5-meter) mirror — larger than any other mirror that's ever flown in space, NASA officials said. (For comparison, the agency's iconic Hubble Space Telescope sports an 8-foot, or 2.4-meter, primary mirror.)
The Webb Telescope 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.
It 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, 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," MIT astrophysicist Sara Seager said during Monday's NASA briefing. "Now nature just has to provide for us." [5 Bold Claims of Alien Life]
A numbers game
But nature may not be so willing, at least during the Webb Telescope's mission. It all comes down to numbers.
There is no shortage of planets in the Milky Way. Our galaxy teems with at least 100 billion planets, 10 to 20 percent of which 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's 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. The Webb Telescope, 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-meter) mirror would give researchers a much better chance of finding biosignatures in alien atmospheres. Mountain would like something even bigger.
"With a 20-meter telescope, we can see hundreds of Earthlike 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, the Webb Telescope is a potentially big step along the way to this goal.
For example, the telescope 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 the Webb Telescope's senior project scientist, John Mather of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Mather won a Nobel Prize in 2006 for his work with the agency's Cosmic Background Explorer satellite.
"Putting together the partnership that can find Earth 2.0 is a challenge worthy of a great generation."
"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 multigenerational effort that goes from TESS and the Webb Telescope to other, larger space telescopes, Seager said. Overcoming the various challenges involved will almost certainly require the cooperation of a number of different countries and organizations.
"Putting together the partnership that can find Earth 2.0 is a challenge worthy of a great generation," Mountain said.
Closer to home
It's possible that alien life will be detected even before humanity launches an enormous space telescope. Confirmation that Earthlings aren't alone in the universe could 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, or JUICE, is 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.