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 the James Webb Space Telescope (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 ($\gtrsim 0.1$ 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 re-examine 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.

Accepted to Astrophysical Journal Letters (in press). 3 Figures, 1 Table in main text; 4 Figures, 5 Tables in SI