Title: Using polarimetry to retrieve the cloud coverage of Earth-like exoplanets,
Abstract: Context. Clouds have already been detected in exoplanetary atmospheres. They
play crucial roles in a planet's atmosphere and climate and can also create
ambiguities in the determination of atmospheric parameters such as trace gas
mixing ratios. Knowledge of cloud properties is required when assessing the
habitability of a planet. Aims. We aim to show that various types of cloud
cover such as polar cusps, subsolar clouds, and patchy clouds on Earth-like
exoplanets can be distinguished from each other using the polarization and flux
of light that is reflected by the planet. Methods. We have computed the flux
and polarization of reflected starlight for different types of (liquid water)
cloud covers on Earth-like model planets using the adding-doubling method, that
fully includes multiple scattering and polarization. Variations in cloud-top
altitudes and planet-wide cloud cover percentages were taken into account.
Results. We find that the different types of cloud cover (polar cusps, subsolar
clouds, and patchy clouds) can be distinguished from each other and that the
percentage of cloud cover can be estimated within 10%. Conclusions. Using our
proposed observational strategy, one should be able to determine basic orbital
parameters of a planet such as orbital inclination and estimate cloud coverage
with reduced ambiguities from the planet's polarization signals along its
orbit.