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When We're Looking for Life on Other Planets, What Exactly Are We Looking For?

Is there life on other planets? To answer that question, you first have to answer this one: what, exactly, is life? We may have a general sense of life as it exists here on Earth -- one that involves a given object's ability to metabolize and grow, to respond to stimuli, to maintain homeostasis, to reproduce. But that textbook definition of life -- "life," that is, "as we know it" -- is itself entirely defined by the particularities of Earth. It's a set of criteria determined by a planet that happens to be terrestrial, that happens to be covered in salt water, that happens to orbit a yellow dwarf star ... and that happens to be shielded from said star by a protective ozone layer. Life as we know it, in other words, is entirely contingent on its environment. As the Harvard astronomy professor Dimitar Sasselov puts it: "We have life here on Earth that has its roots in the chemistry of the planet." This raises a challenge for astrobiologists: if we're going to try to answer the big is-there-life-out-there question ... how, exactly, should we go about it? When it comes to the universe at large, "we don't have a definition of life, and we don't know what it takes for life to emerge," Sasselov noted in a talk at the Aspen Ideas Festival yesterday evening. "So how do we know that life on other planets will be similar to what we have here?" via The Atlantic.

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