2022ApJ...930..131R

About 2.5 billion years ago, microbes learned to harness plentiful solar energy to reduce CO₂ with H₂O, extracting energy and producing O₂ as waste. O₂ production from this metabolic process was so vigorous that it saturated its photochemical sinks, permitting it to reach "runaway" conditions and rapidly accumulate in the atmosphere despite its reactivity. Here we argue that O₂ may not be unique: diverse gases produced by life may experience a "runaway" effect similar to O₂. This runaway occurs because the ability of an atmosphere to photochemically cleanse itself of trace gases is generally finite. If produced at rates exceeding this finite limit, even reactive gases can rapidly accumulate to high concentrations and become potentially detectable. Planets orbiting smaller, cooler stars, such as the M dwarfs that are the prime targets for the James Webb Space Telescope (JWST), are especially favorable for runaway, due to their lower UV emission compared to higher-mass stars. As an illustrative case study, we show that on a habitable exoplanet with an H₂-N₂ atmosphere and net surface production of NH₃ orbiting an M dwarf (the "Cold Haber World" scenario), the reactive biogenic gas NH₃ can enter runaway, whereupon an increase in the surface production flux of one order of magnitude can increase NH₃ concentrations by three orders of magnitude and render it detectable by JWST in just two transits. Our work on this and other gases suggests that diverse signs of life on exoplanets may be readily detectable at biochemically plausible production rates.