Characterizing Earth-like Exoplanets: Insights from Earthshine Observations

The next generation of ground- and space-based telescopes, such as ANDES and PCS at the ELT and the mission concept Habitable World Observatory (HWO), will enable the study of exoplanets in reflected light, extending this capability to rocky planets. To better understand Earth as an exoplanet, we analyze its spatially unresolved visible reflected spectrum via the sunlight reflected by Earth onto the Moon (Earthshine). Using the 3D Monte Carlo radiative transfer code MYSTIC, we generate synthetic spectra and phase curves of Earth in both intensity and polarization, incorporating realistic 3D atmospheric pressure-temperature profiles, patchy clouds, and wavelength-dependent surface albedo maps. By comparing these simulations with Earthshine data, we evaluate the sensitivity of key spectral features for planetary characterization and habitability assessment. Our findings highlight the necessity of detailed atmospheric and surface albedo modeling to accurately reproduce Earthshine observations. These insights are essential for future missions targeting Earth-like exoplanet characterization. By advancing our understanding of Earth's radiative and spectral properties, we establish a basis for interpreting observations of similar distant planets. We also formulate an optimal strategy, through a comparison of spectroscopy and spectropolarimetry, for assessing the chemical diversity of Earth-like exoplanets with upcoming telescopes.