The Search for Biosignature Gases on Exoplanets
Sara Seager
Massachusetts Institute of Technology

Thousands of exoplanets are known to orbit nearby stars and small rocky planets are established to be common. The ambitious goal of identifying a habitable or inhabited world is within reach. But how likely are we to succeed? We need to first discover a pool of planets in their host star's "extended" habitable zone and second observe their atmospheres in detail to identify the presence of water, a requirement for all life as we know it. Life must not only exist on one of those planets, but the life must produce "biosignature gases" that are spectroscopically active, and we need to be able to sort through a growing list of false-positive scenarios with what is likely to be limited data. The race to find habitable exoplanets has accelerated with the realization that "big Earths" fortuitously aligned to transit small stars can be both discovered and characterized with current technology, such that the James Webb Space Telescope (launch 2018) has a chance to be the first to provide evidence of biosignature gases. What will it take to identify such habitable worlds, amidst a yet unknown range of planetary environments, with the observations and theoretical tools available to us?