10:15 AM - 10:30 AM
[PPS07-23] The effect of spectral type of central star on climate and climatic evolution of the Earth-like planets in habitable zone
Keywords:Exoplanet, Carbonate-silicate geochemical cycle, Habitable planet
We use a one-dimensional energy balance model coupled with a carbon cycle model to estimate the climate, and the planetary albedo model is improved in order to examine the effect of the difference in the spectrum of the insolation from the central star. The evolution of the climate is examined based on the evolutions of CO2 degassing rate and insolation, which are estimated by a parameterized convection model coupled with a mantle degassing model and a luminosity evolution model, respectively. Four types of stars (i.e., M-, K-, G-, and F-type stars) are considered.
Comparing stars with different mass (e.g., M- and G-type stars), pCO2 of an Earth-like planet around a light star (i.e., the M-type star) tends to be lower than that of an Earth-like planet around a heavy star (i.e., G-type star) for the same luminosity and CO2 degassing rate. This is because the peak wavelength of the insolation of the light star is longer than that of the heavy star, and because the ice absorbs longer-wavelength radiation more than shorter-wavelength radiation. As a result, the critical CO2 degassing rate is less in the inner region of the HZ around the light star than in the region around the heavy star. However, when the Earth-planet is in the outer region of the HZ, and pCO2 is high owing to the Walker feedback, the critical CO2 degassing rate of the Earth around the light star is almost the same as that of the Earth around the heavy star especially in the outer region of the HZ because the surface albedo does not affect the planetary albedo owing to the dense atmosphere. Thus, regardless of the spectral type of the central star, the timescale for the warm climate of Earth-like planet is about 4 billion years which depends, not on the insolation, but strongly on the evolution of the CO2 degassing rate of the planet. These results indicate that we should search for the inner region of the HZ around young stars to find Earth-like habitable planets.