Vol. 548
In section 9. The Sun

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

by T. Karalidi, D. M. Stam, and J. W. Hovenier, A&A 548, A90

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Detecting and identifying another habitable planet with liquid water at its surface is a challenging task, but we may get unexpected help from rainbows! Rainbows are caused by the reflection and diffraction of stellar light by water droplets in suspension in the atmosphere. Although the beautiful color dispersion effect leading to the rainbow itself would not be detectable on an exoplanet, it is also accompanied by a phase-dependent reflection of the stellar light and by strong polarization, both of which may be detectable given the proper instrument, sensitivity, and observing strategy. Using a detailed multiple-scattering radiative transfer model and data for our Earth, the authors study whether this effect may be masked by ice clouds or may require a large portion of the planetary surface to be covered by liquid water clouds. Indeed, the characteristic signature of water clouds on the reflected starlight flux as a function of phase angle may be masked by the presence of ice clouds. However, the polarization signal appears to be a much more robust signature of the presence of water clouds: even when only 25% of the planet is covered by water clouds and these are covered at 50% by ice clouds (a pessimistic case when compared to the situation for the Earth), a low peak in the polarization remains present at a 30° planetary phase angle. Polarization measurements should thus be considered in future missions for characterizing exoplanets.