| Abstract : | To recognize a habitable exoplanet from observed spectra, we model the reflected spectra for modern and prebiotic (3.9 Ga) Earth-like exoplanets orbiting at the habitable zone of stars of F, G, K, and M spectral types for different zenith angles of incident stellar radiation. Specific cases of Earth like planets like Proxima Centauri b, Trappist-1e, Kepler-442b, Kepler-62e, and Kepler-22b are also included. We also estimate the geometric albedo for modern Earth-like exoplanets with an increased abundance of greenhouse gases. We employ the opacity data derived by using the open-source package ”Exo-Transmit” and adopt different temperature-pressure profiles depending on the planet. The multiple scattering Radiative Transfer Equations are solved to generate the reflected spectra. We compare our model reflected spectra with the observed reflected spectrum of modern Earth and theoretical spectra for prebiotic Earth. We also verify our results for the case of Trappist-1e. We find the atmospheric spectral features for the above planets. These planets can be the targets for the upcoming reflected light observations. We demonstrate that the present Earth-like exoplanets with an increased abundance of greenhouse gases in their atmosphere and the prebiotic Earth-like exoplanets scatter more starlight in the optical compared to the planets having atmospheric composition similar to that of the present Earth. The reflected flux increases with surface albedo and decreases with the increase in zenith angle. We also calculate the transmission depth for modern and early Earth like exoplanets, which increases with the abundance of greenhouse gases, although it decreases with the increase in cloud height from the ground. |
