Will we know extraterrestrial life when we see it?

Recognizing life on other worlds requires wiggle room in the definition of what it means to be alive

illustration of alien microbe

LIFE ON MARS  A Martian microbe, envisioned by planetary geologist Kathie Thomas-Keprta, would need a tough outer wall to withstand the elements and magnetic crystals to help it navigate.

K. Thomas-Keprta/NASA Johnson Space Ctr.

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