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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

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12:00pm, April 18, 2016
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.

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In a 1967 episode of Star Trek, Captain Kirk and crew investigated the mysterious murders of miners on the planet Janus VI. The killer, it turned out, was a rock monster called the Horta. But the Enterprise’s sensors hadn’t registered any signs of life in the creature. The Horta was a silicon-based life-form, rather than carbon-based like living things on Earth.

Still, it didn’t take long to determine that the Horta was alive. The first clue was that it skittered about. Spock closed the case with a mind meld, learning that the creature was the last of its kind, protecting its throng of eggs.

But recognizing life on different worlds isn’t likely to be this simple, especially if the recipe for life elsewhere doesn’t use familiar ingredients. There may even be things alive on Earth that have been overlooked because they don’t fit standard definitions of life, some scientists

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