Characterizing rocky exoplanets through their reflected and polarized light

Understanding what makes a planet habitable requires learning how to interpret the faint light reflected by distant worlds. Crucially, the polarization state of reflected light carries an almost unique fingerprint of the scattering processes that shape it, discriminating cloud particles, surface types, and atmospheric composition where photometry and spectroscopy alone remain degenerate.

Reflected-light observations will be central to detecting and characterizing rocky exoplanets, and instruments such as ANDES and PCS on the ELT will soon make it possible to observe non-transiting planets like Proxima b. To prepare for such observations, we study Earth as an exoplanet through Earthshine - the sunlight reflected by Earth onto the dark portion of the visible Moon, capturing the planet as a single, unresolved pixel.

Using 3D radiative transfer simulations, we model Earth's atmosphere and surface, computing both the reflected and polarized light spectrum and phase curves. Comparing simulations with spectropolarimetric Earthshine observations, we show how polarized signatures, such as the ocean glint and the primary rainbow, can reveal the presence of liquid water on the planet. In this talk, I will show how Earth provides a crucial benchmark for the characterization of rocky exoplanets, guiding future observations with the ELT and HWO.