2:00 PM - 2:15 PM
[PEM11-12] Theoretical study of climates on Earth-like and tidally locked planets around the outer edge of the habitable zone
Keywords:exoplanet, climate, habitable zone
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.
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.