[U-11] Biogeochemistry of CO world

[U11-02] How to find CO worlds?

*Kazumi Ozaki¹ (1.Tokyo Institute of Technology)

Keywords:Biogeochemical modeling, Carbon moonxide, Exoplanet

The abundances of major carbon species (CO₂, CO, and CH₄) in the atmosphere exert fundamental controls on planetary climate and biogeochemistry. Watanabe and Ozaki (2024) employed a theoretical model of atmospheric photochemistry to investigate the factors controlling relative abundances of CO₂, CO, and CH₄ in the atmospheres of Earth-like lifeless planets orbiting Sun-like (F, G, and K-type) stars, with particular attention to the conditions for CO runaway—a photochemical instability of CO budget in the atmosphere. Their systematic experiments demonstrate that elevated CO₂ levels and/or higher degassing rates of reducing gases from the planetary interior tend to trigger CO runaway. Based on the theoretical framework of the carbonate-silicate geochemical cycle, habitable planets in the early stages of planetary systems (characterized by smaller central star luminosities) tend to have higher atmospheric CO₂ levels and are more likely to form CO-rich atmospheres. Similar conclusions can be drawn for planets located near the outer boundary of the habitable zone. The sensitivity experiments with respect to the stellar spectral type also indicate that the formation of CO-rich atmospheres is relatively difficult on a planet orbiting F-type stars and relatively easy on one orbiting K- and M-type stars. In short, habitable planets orbiting G-, K-, or M-type young stars (especially those located near the outer boundary of the habitable zone) are preferable candidates to search for CO worlds. The future direction and remaining problems will also be discussed.