The Importance of the Upper Atmosphere to CO/O 2 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 O 2 and CO, which are potential indicators of life. Recently, it was proposed that CO 2 photolysis generates abundant (≳0.1 bar) abiotic O 2 and CO in the atmospheres of habitable M dwarf planets with CO 2 -rich atmospheres, constituting a strong false positive for O 2 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 O 2 and CO, if habitable. Here, we use a multi-model approach to reexamine photochemical O 2 and CO accumulation on planets orbiting M dwarf stars. We show that photochemical O 2 remains a trace gas on habitable CO 2 -rich M dwarf planets, with earlier predictions of abundant O 2 and CO due to an atmospheric model top that was too low to accurately resolve the unusually high CO 2 photolysis peak on such worlds. Our work strengthens the case for O 2 as a biosignature gas, and affirms the importance of CO as a diagnostic of photochemical O 2 production. However, observationally relevant false-positive potential remains, especially for O 2 's photochemical product O 3 , and further work is required to confidently understand O 2 and O 3 as biosignature gases on M dwarf planets.