Earth's cousins: Upcoming missions to look for 'biosignatures' in exoplanet atmospheres

Scientists have discovered thousands of exoplanets, including dozens of terrestrial--or rocky--worlds in the habitable zones around their parent stars. A promising approach to search for signs of life on these worlds is to probe exoplanet atmospheres for "biosignatures"--quirks in chemical composition that are telltale signs of life. For example, thanks to photosynthesis, our atmosphere is nearly 21% oxygen, a much higher level than expected given Earth's composition, orbit and parent star.