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 O2β and CO, which are potential indicators of life. Recently, it was
proposed that CO2β photolysis generates abundant (β³0.1 bar) abiotic
O2β and CO in the atmospheres of habitable M-dwarf planets with CO2β-rich
atmospheres, constituting a strong false positive for O2β as a biosignature
and further complicating efforts to use CO as a diagnostic of surface biology.
Significantly, this implied that TRAPPIST-1e and TRAPPIST-1f, now under
observation with JWST, would abiotically accumulate abundant O2β and CO, if
habitable. Here, we use a multi-model approach to re-examine photochemical
O2β and CO accumulation on planets orbiting M-dwarf stars. We show that
photochemical O2β remains a trace gas on habitable CO2β-rich M-dwarf
planets, with earlier predictions of abundant O2β and CO due to an
atmospheric model top that was too low to accurately resolve the unusually-high
CO2β photolysis peak on such worlds. Our work strengthens the case for O2β
as a biosignature gas, and affirms the importance of CO as a diagnostic of
photochemical O2β production. However, observationally relevant false
positive potential remains, especially for O2β's photochemical product O3β,
and further work is required to confidently understand O2β and O3β as
biosignature gases on M-dwarf planets.Comment: Submitted to AAS Journals; comments and criticism solicited at
[email protected]. 3 Figures, 1 Table in main text; 3Figures, 5 Tables in S