Uncovering Exoplanets using Polarimetry

Since the first discovery of a planet around a solar-type star by Mayor & Queloz in 1995, more than 700 of these exoplanets have been detected. Most of these are giant, gaseous planets, but small, presumably solid, exoplanets, that are much harder to detect, have also been found. Among the latter are even some that orbit in their star's habitable zone, where temperatures could be just right to allow liquid water on a planet's surface. Liquid water is generally considered to be essential for the existence of life. Whether liquid water actually exists on a planet depends strongly on the atmosphere's thickness and characteristics, such as the surface pressure and composition. Famous examples in the Solar System are Venus and the Earth, with similar sizes, inner compositions and orbital radii, but wildly different surface conditions. The characterization of the atmospheres of giant, gaseous exoplanets, and of the atmospheres and/or surfaces of small, solid exoplanets will allow a comparison with Solar System planets and it will open up a treasure trove of knowledge about the formation and evolution of planetary atmospheres and surfaces, thanks to the vast range of orbital distances, planet sizes and ages that can be studied. Characterization will also allow studying conditions for life and ultimately the existence of life around other stars. Some information about the upper atmospheric properties has already been derived for a few close-in, hot, giant exoplanets, whose thermal flux can be derived from measurements of the combined flux of the star and the planet. This method has also provided traces of an atmosphere around a large solid planet orbiting red dwarf star GJ1214. Characterization of the atmosphere and/or surface of exoplanets in wide orbits, resembling the cool planets in our Solar System, and in particular of small, solid, Earth-like planets in the habitable zone of Sun-like stars, is virtually impossible with transit observations. Indeed, polarimetry appears to be a strong tool both for the detection and the characterization of such cool exoplanets. Polarimetry helps their detection, because direct starlight is usually unpolarized, while starlight that has been reflected by a planet is usually polarized, especially at the phase angles favorable for observing exoplanets. Polarimetry thus improves the contrast between stars and their planets, and confirms that the detected object is indeed a planet. In my presentation, I will focus on the power of polarimetry for the characterization of exoplanets. This application is known from the derivation of the Venus cloud properties from the planet's polarized phase function by Hansen & Hovenier in 1974. Using numerically simulated flux and polarization phase functions and spectra for both gaseous and solid exoplanets, I will discuss the added value of polarimetry for exoplanet characterization as compared to flux observations, in particular for the retrieval of properties of clouds and hazes. Special attention will be given to the features in polarized phase functions that reveal the existence of liquid water clouds in the atmosphere (rainbows), even in the presence of ice clouds, or liquid water on the surface (glint) of an exoplanet. Using satellite data of the cloud and surface coverage of the Earth, calculated flux and polarization phase functions that should be observable from afar will be presented.