The Importance of the Upper Atmosphere to CO/O₂ Runaway on Habitable Planets Orbiting Low-mass Stars

Abstract

Efforts to spectrally characterize the atmospheric compositions of temperate terrestrial exoplanets orbiting M dwarf stars with JWST are now underway. Key molecular targets of such searches include O₂ and CO, which are potential indicators of life. Recently, it was proposed that CO₂ photolysis generates abundant (≳0.1 bar) abiotic O₂ and CO in the atmospheres of habitable M dwarf planets with CO₂-rich atmospheres, constituting a strong false positive for O₂ as a biosignature and further complicating efforts to use CO as a diagnostic of surface biology. Importantly, this implied that TRAPPIST-1e and TRAPPIST-1f, now under observation with JWST, would abiotically accumulate abundant O₂ and CO, if habitable. Here, we use a multi-model approach to reexamine photochemical O₂ and CO accumulation on planets orbiting M dwarf stars. We show that photochemical O₂ remains a trace gas on habitable CO₂-rich M dwarf planets, with earlier predictions of abundant O₂ and CO due to an atmospheric model top that was too low to accurately resolve the unusually high CO₂ photolysis peak on such worlds. Our work strengthens the case for O₂ as a biosignature gas, and affirms the importance of CO as a diagnostic of photochemical O₂ production. However, observationally relevant false-positive potential remains, especially for O₂'s photochemical product O₃, and further work is required to confidently understand O₂ and O₃ as biosignature gases on M dwarf planets.

Files

Additional details