Radio dishes pointed skyward at the Australian Telescope Compact Array. Radio astronomy is the focus of the search for extraterrestrial intelligence, or SETI. (CSIRO)

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?"

The short answer is that we don't. We have to rely on educated guesses based on our own sense of "life," since we simply don't know exactly what life outside of that framework might look like. (Or act like, or smell like, or think like, or something-that-we-don't-even-have-a-word-for like.) We talk about the "habitable zone," the region around a given star that potentially gives the planets within it favorable conditions for surface water. And we focus our scientific investigations on planets within that zone. We look, basically, for exo-Earths -- for planets that could have ostensibly supported life in the same way Earth does. And we're finding them.

It's an approach that is purposely narrow and, in that, practical. And it's one that leaves some big questions -- yes, purposely -- unanswered. Our sun, Sasselov pointed out, is a yellow dwarf; yet the most common type of star in the galaxy and in observable universe, so far as we can tell, is the red dwarf. "We're orbiting a very uncommon star," he said. So "what does this tell us about life?" How unique, actually, is Earth? And how unique are its inhabitants?

We will, Sasselov believes, soon find out. The James Webb Space Telescope, currently under construction and slated for launch in 2018, will train its lens on the earliest galaxies that formed in the universe. The Giant Magellan Telescope, expected to be seeing first light in 2020, will focus on extrasolar planets -- with a stated goal of searching for evidence of life in other solar systems. Sasselov and his team at Harvard's Origins of Life Initiative are currently doing work that takes the knowledge we've gained from 50 years of genetics research and applies it to the origins of life itself. For the first time, the professor said, we're capable of synthesizing the molecules that may have been present on Earth when life first formed on the planet. And that synthetic approach gives the team a chemical framework for understanding how life might have formed in other chemical contexts. As Sasselov put it, "We're trying to understand the basics of life based on environments that are different from what we know on Earth."

So while we currently embrace a necessarily narrow -- and pragmatically self-centered -- view of life, there's a chance that new discoveries will expand our sense of what life itself can look like. "Once every few centuries, we have a major shift in the human frame of reference," Sasselov said." Our explorations of solar systems and galaxies far beyond our own, coupled with our expanding knowledge of the planet we call home, could well lead to the next one.