Colloquium: In Pursuit of the Elusive: The Search for Exoplanet Biosignature Gases
The search for signs of life beyond Earth is a key motivator in exoplanet research. A suitable “biosignature gas” is one that: can accumulate in an atmosphere against atmospheric radicals and other sinks; has strong atmospheric spectral features; and has limited abiological false positives. We now have a growing list of potential biosignature gases including isoprene which is produced by life in as high quantities as methane, and phosphine which on Earth is only associated with life. Despite promise from the successfully operational JWST, we are now confronted with practical challenges of tiny signals and M dwarf star contamination. Another severe challenge is the unknown exoplanetary environments likely vastly different from Solar System planets. We review the thousands of molecules produced by life on Earth in context with the phosphine on Venus and methane on Mars prescient backdrop for a reality check on the future of exoplanet biosignature gases. Our pace and history of milestone discovery in exoplanets in the last three decades, combined with new telescope paradigms, promises to eventually deliver on finding signs of life beyond Earth.