2020ApJ...888..122H

Small exoplanets of nearby M-dwarf stars present the possibility of finding and characterizing habitable worlds within the next decade. TRAPPIST-1, an ultracool M-dwarf star, was recently found to have seven Earth-sized planets of predominantly rocky composition. The planets e, f, and g could have a liquid water ocean on their surface given appropriate atmospheres of N₂ and CO₂. In particular, climate models have shown that the planets e and f can sustain a global liquid water ocean, for >=0.2 bar CO₂ plus 1 bar N₂, or >=2 bar CO₂, respectively. These atmospheres are irradiated by ultraviolet emission from the star's moderately active chromosphere, and the consequence of this irradiation is unknown. Here we show that chemical reactions driven by the irradiation produce and maintain more than 0.2 bar O₂ and 0.05 bar CO if the CO₂ is >=0.1 bar. The abundance of O₂ and CO can rise to more than 1 bar under certain boundary conditions. Because of this O₂-CO runaway, habitable environments on the TRAPPIST-1 planets entail an O₂- and CO-rich atmosphere with coexisting O₃. The only process that would prevent runaway is direct recombination of O₂ and CO in the ocean, a reaction that is facilitated biologically. Our results indicate that O₂, O₃, and CO should be considered together with CO₂ as the primary molecules in the search for atmospheric signatures from temperate and rocky planets of TRAPPIST-1 and other M-dwarf stars.