O 2 - and CO-rich Atmospheres for Potentially Habitable Environments on TRAPPIST-1 Planets

Abstract

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 N 2 and CO 2 . In particular, climate models have shown that the planets e and f can sustain a global liquid water ocean, for ≥0.2 bar CO 2 plus 1 bar N 2 , or ≥2 bar CO 2 , 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 O 2 and 0.05 bar CO if the CO 2 is ≥0.1 bar. The abundance of O 2 and CO can rise to more than 1 bar under certain boundary conditions. Because of this O 2 –CO runaway, habitable environments on the TRAPPIST-1 planets entail an O 2 - and CO-rich atmosphere with coexisting O 3 . The only process that would prevent runaway is direct recombination of O 2 and CO in the ocean, a reaction that is facilitated biologically. Our results indicate that O 2 , O 3 , and CO should be considered together with CO 2 as the primary molecules in the search for atmospheric signatures from temperate and rocky planets of TRAPPIST-1 and other M-dwarf stars.