日本地球惑星科学連合2023年大会

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[E] 口頭発表

セッション記号 P (宇宙惑星科学) » P-EM 太陽地球系科学・宇宙電磁気学・宇宙環境

[P-EM11] 系外惑星

2023年5月24日(水) 13:45 〜 15:15 102 (幕張メッセ国際会議場)

コンビーナ:小玉 貴則(東京大学)、野津 翔太(理化学研究所 開拓研究本部 坂井星・惑星形成研究室)、川島 由依(理化学研究所)、森 万由子(東京大学)、座長:中山 陽史(立教大学理学研究科物理学専攻)、小玉 貴則(東京大学)

14:00 〜 14:15

[PEM11-12] Theoretical study of climates on Earth-like and tidally locked planets around the outer edge of the habitable zone

*谷口 啓悟1,2玄田 英典1小玉 貴則3 (1.東京工業大学 地球生命研究所、2.東京工業大学 理学院 地球惑星科学系、3.東京大学 先進科学研究機構)


キーワード:系外惑星、気候、ハビタブルゾーン

Habitable zone (HZ) is a virtual region around the host star where a terrestrial planet can maintain liquid water on its surface (Kasting et al., 1993). Especially, the outer edge of the HZ is defined as the distance where the CO2 greenhouse effect on a planet becomes maximum. According to the previous study (Kopparapu et al., 2013), the outer edge of the HZ is 35 % of solar constant, or 1.69 AU from the Sun, when the greenhouse effect reaches maximum (8 bar).

The HZ around M-type stars is closer to the host star because of lower luminosity. In addition, the planet around M-type stars is likely to be tidally locked. Therefore, the same hemisphere faces the central star all the time (day-side or night-side hemisphere). If the night-side surface temperature is extremely low because of no irradiation, volatile species such as CO2 and CH4 can easily condense on the surface (e.g., Turbet et al., 2018). This is called atmospheric collapse and is considered as an obstruction to a warm climate due to the weakening of greenhouse effects.

In our research, we examined the conditions for atmospheric collapse on tidally locked Earth-like planets, and how atmospheric collapse affects the outer edge of the HZ. We used a global climate model (GCM) developed by LMD-IPSL. We calculated climates for various atmospheric conditions by changing pN2 and pCO2 as in Turbet et al. (2018). The initial condition is completely ice-covered planet, and the onset of atmospheric collapse is determined by whether the minimum surface temperature is lower than the sublimation temperature of CO2. As a result, we found that the atmosphere with pCO2 higher than 1 mbar easily collapses. Besides, we investigated the pCO2 after the atmospheric collapse and found the cases that the post-collapse climate has a local liquid water area on the day-side, although the pre-collapse planet is snowball. This is because the day-night heat transport becomes weak due to a decrease in the total amount of atmosphere and energy distribution on the day-side.

According to the traditional theory, the definition of outer edge requires a massive CO2 atmosphere. However, this study suggests that the atmospheric collapse prohibits a large amount of CO2. On the other hand, atmospheric collapse on tidally locked planets can provide rather a locally habitable planet because of weakened heat transport. This result differs from the conclusions considered in the traditional concept, such as ancient Mars. Our results could change the previously considered conditions of HZ and atmospheric collapse. At the same time, this provides a new picture of the climate on tidally locked planets. The constraints we derived could serve as a guideline to current/future telescope observations such as JWST and JASMINE.