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.