Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures

TL;DR

This paper explores how certain gases produced by life on exoplanets can undergo a runaway accumulation in atmospheres, making them detectable biosignatures, especially around M dwarf stars, with implications for future observations.

Contribution

It introduces the concept of photochemical runaway for biosignature gases, showing how reactive gases can accumulate rapidly under certain conditions, expanding the scope of detectable signs of life.

Findings

Runaway accumulation of gases like NH₃ is possible in exoplanet atmospheres.

Lower UV environments around M dwarfs favor gas buildup and detection.

A 10x increase in surface production can boost NH₃ levels by 1000 times, making it detectable in two JWST transits.

Abstract

About 2.5 billion years ago, microbes learned to harness plentiful solar energy to reduce CO$_2$ with H$_2$O, extracting energy and producing O$_2$ as waste. O$_2$ 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 O$_2$ may not be unique: diverse gases produced by life may experience a "runaway" effect similar to O$_2$. 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 H$_2$-N$_2$ atmosphere and net surface production of NH$_3$ orbiting an M dwarf (the "Cold Haber World" scenario), the reactive biogenic gas NH$_3$ can enter runaway, whereupon an increase in the surface production flux of one order of magnitude can increase NH$_3$ 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.