How do we look for life on other planets when we have only the single example of life on earth to inform our search? That question is so challenging that the National Academies of Science convened an expert panel to try to answer the question. The World Science Festival had a panel of its own on the topic, consisting of Steve Benner, Paul Davies, and Margaret Turnbull. Turnbull has almost no web presence, but is involved in planning space-based observatories designed to observe extrasolar planets.
Davies gave a great explanation of why the problem is so hard. Most definitions of life include a list of properties it has, and nearly every one of them can be found in nonliving matter; it's really the combination that's distinctive. Benner argued that our definitions are unduly influenced by the biological theories we have, noting that even one of the best—'a self sustaining chemical system capable of Darwinian evolution'—is "laden with our theories." Davies more or less concurred, pointing out that, even though the extremophiles we've found on earth tell us a bit about what's possible, they still act more or less like us, with ribosomes, DNA, etc. Still, this might be helpful, as evolutionary convergence suggests that some physical or chemical features of earth life might have analogs elsewhere.
What might non-earth life look like? Benner described creating a 12-base DNA system that worked in the lab, although graduate students had to continue to feed the artificial bases to the organisms. Going further afield, Davies described how lots of people have been excited by the prospect of some form of life catalyzed on the surface of clay, but suggested things of that nature should still be happening in the wild, and they're apparently not. "Even if it's happening," he said, "it's not doing anything especially interesting."
After Benner and Turnbull agreed that water was probably a prerequisite, Davies averred, "I'm not a big fan of water." Asked for a solvent with a similar set of properties, he suggested formamide. Davis was a fan of chirality, suggesting that biochemistry needs a bias in the handedness of molecules, so looking for that could help us spot life, even if we aren't sure of its actual chemical basis. It seems that a chiral bias could also help us identify the chemical processes unfamiliar life was engaging in, although Davies didn't mention this specifically.
The discussion also turned to the key question of instrumentation. Davies argued that the microscope completely changed our concept of what life on earth was like, since it revealed a vast single-celled world. After Benner mused that the new microscope is the telescope, Turnbull took over, since she designs them for a living. She describes how NASA's planned Kepler orbiting observatory will be able to detect an earth-like planet transiting in front of a star; if it works, then follow-on observatories may be able to get spectra of its atmosphere; she mentioned one potential design that included a specially designed sunshade orbiting 80,000 km from a four meter telescope.
With the possibility to observe the components and changes in a planet's atmosphere, we can watch for things like water (or formamide), hints of tectonic activity, potential energy gradients that could be harvested by life, etc. We wouldn't know whether there was life there, but we could definitely identify places with favorable conditions.
So, are there any laws of life? Turnbull was skeptical. "We're just trying to put things in boxes, and it's really a continuum," she said, and argued that there won't be anything like a phase change between living and nonliving. Benner said the laws we have—cell, gene, and evolutionary theories—aren't like the laws of physics. "If we really understand them, then we can repeat the process in the lab," he suggested, "we're not there yet." Davies' take is that "biology doesn't have theorists in the same way that physics does, possibly because it's all history and contingency." Until we find a second example of life, it's hard to separate the basics from the contingencies.
Overall, the takehome message seemed to be that the question itself was a really hard one, and we can expect the answers to change as we get a better grip on biology and origin of life issues in the coming years.