Looking for the rainbow on exoplanets covered by liquid and icy water clouds

TL;DR

This study investigates the detectability of the primary rainbow in reflected starlight from exoplanets with liquid and icy water clouds, highlighting polarization as a key detection method even with partial cloud coverage.

Contribution

It demonstrates that polarization signals can reveal liquid water clouds on exoplanets despite ice cloud masking, using realistic Earth-like cloud coverage models.

Findings

Polarization enhances rainbow detection even with partial cloud coverage.

Ice clouds dampen the rainbow feature in total flux but not in polarization.

Realistic Earth-like cloud models show strong polarized rainbow signals.

Abstract

Looking for the primary rainbow in starlight that is reflected by exoplanets appears to be a promising method to search for liquid water clouds in exoplanetary atmospheres. Ice water clouds, that consist of water crystals instead of water droplets, could potentially mask the rainbow feature in the planetary signal by covering liquid water clouds. Here, we investigate the strength of the rainbow feature for exoplanets that have liquid and icy water clouds in their atmosphere, and calculate the rainbow feature for a realistic cloud coverage of Earth. We calculate flux and polarization signals of starlight that is reflected by horizontally and vertically inhomogeneous Earth--like exoplanets, covered by patchy clouds consisting of liquid water droplets or water ice crystals. The planetary surfaces are black. On a planet with a significant coverage of liquid water clouds only, the total flux signal shows a weak rainbow feature. Any coverage of the liquid water clouds by ice clouds, however, dampens the rainbow feature in the total flux, and thus the discovery of liquid water in the atmosphere. On the other hand, detecting the primary rainbow in the polarization signal of exoplanets appears to be a powerful tool for detecting liquid water in exoplanetary atmospheres, even when these clouds are partially covered by ice clouds. In particular, liquid water clouds covering as little as 10%-20% of the planetary surface, with more than half of these covered by ice clouds, still create a polarized rainbow feature in the planetary signal. Indeed, calculations of flux and polarization signals of an exoplanet with a realistic Earth--like cloud coverage, show a strong polarized rainbow feature.