講演情報
[U17-01]Stellar control on atmospheric carbon chemistry, CO runaway, and organic synthesis on lifeless Earth-like planets
遠藤 美朗1、渡辺 泰士2、*尾崎 和海1 (1.東京科学大学、2.国立環境研究所)
キーワード：
COワールド、大気、バイオシグネチャー、理論モデル
The atmospheric composition of terrestrial planets within the habitable zone (HZ) is shaped by tightly coupled interactions among photochemistry, climate, volcanic outgassing, and the long-term carbon cycle. While CO2 has been extensively studied as a key regulator of planetary climate, much less is known about how the relative abundances of CO2, CO, and CH4 vary systematically across the HZ in the absence of biology, and how these trends depend on stellar spectral type. Clarifying these relaionships is essential for interpreting future obsevations of Earth-sized exoplanets and for assessing environments favorable for the origin of life. Here we investigate the atmospheric compositions of lifeless, Earth-like planets orbiting Sun-like (F-, G-, and K-type) stars using an integrated atmospheric chemistry-climate-carbon cycle model. Our simulations demonstrate that CO2, CO, and CH4 generally increase with orbital distance, and that planets near the outer edge of the HZ may undergo CO runaway—a photochemical instability driven by severe depletion of OH radicals. The threshold for CO runaway depends strongly on stellar spectral type and is most easily triggered around cooler, lower-mass stars. In contrast, the atmospheric production of formaldehyde (H2CO)—a key precursor for prebiotic organic chemistry—peaks around planets orbiting more massive, UV-luminous stars and is maximized at orbital distances just interior to the CO-runaway threshold. By quantitatively linking stellar spectral type and orbital distance to atmospheric carbon chemistry, our study provides a physically grounded framework for interpreting CO and CO/CH4 as potential anti-biosignatures and for identifying planetary systems in which prebiotic chemical environments are most likely to arise. These findings have important implications for target selection and data interpretation in upcoming missions aimed at characterizing temperate terrestrial exoplanets.