Impacts of Atmospheric Carbon Species and Stellar Type on Climates of Terrestrial Planets

Abstract The climates of terrestrial planets are largely determined by the composition of their atmospheres and the spectral types of their host stars. Previous studies suggest that a wide range of carbon species abundances (CO 2 , CO, and CH 4 ) can result from variations in reducing fluxes and stellar spectral types that influence photochemistry. However, a systematic investigation of how varying carbon species, particularly CO, affect planetary climates across wide parameter spaces remains limited. Here, we employ a 1D radiative–convective equilibrium model to examine the dependence of planetary climate on the abundances of carbon species and host star type. We find that CO, due to weak absorption of stellar radiation, induces only moderate changes in stratospheric temperature, while its effect on surface temperature is negligible. Under Earth-like <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> (where p i is the partial pressure on the surface of species i ), for cases with fixed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> , an increase in CO leads to surface cooling on planets orbiting Sun-like stars unless the sum of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CH</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> exceeds ∼1 bar, whereas it results in surface warming for planets around M-type stars. When the total pressure of carbon species is fixed, converting CO 2 or CH 4 into CO always induces cooling. These effects arise from a combination of CO Rayleigh scattering, pressure broadening of greenhouse gas absorption lines, and varying water vapor levels. We further discuss how CO- and CH 4 -driven cooling (warming) can trigger positive (negative) climate-photochemistry feedback, influencing atmospheric evolution. Additionally, we suggest that CO-rich planets may be less susceptible to water loss and atmospheric oxidation due to lower stratospheric water vapor content.