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A visit to Jupiter's moon Europa in the 2020s is among the upcoming NASA missions that have real potential to search for "biosignatures," or substances that provide evidence of life. NASA/JPL hide caption

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If You're Looking For Alien Life, How Will You Know If You've Found it?

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Don't go there: A warning sign marks the boundary of Area 51 in Nevada. Dan Callister/Getty Images hide caption

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Dan Callister/Getty Images