Interpreting condensate contributions to reflected light signatures of hot Jupiters through polarimetry

Polarimetry is uniquely suited to determine and confirm key properties of condensates, from droplet size and shape, to identifying the condensate species (see Bailey 2007), to detecting asymmetry in cloud distribution (e.g. Shalygina et al 2008), and in discerning cloud decks from the light reflected off a surface (Fauchez et al. 2017). The approach benefits exoplanets by allowing a means to detect the light reflected from the planet without the need to “subtract” the host star’s light or resolve the planet. However, the detection of polarized light reflected from an exoplanet has only yielded tentative detections thus far (Berdyugina et al. 2011; Wiktorowicz 2009; Wiktorowicz et al. 2015; Bott et al. 2016). Though the field is encroaching on the sensitivity and light collection needed to make these detections, and similar observations have recently successfully described the condensate structure and oblateness of brown dwarfs (Millar-Blanchaer et al. 2020, Jensen-Clem et al. 2020, Bryan et al. 2020), the effects of condensates on a polarized light curve must be taken into consideration in the interpretation of exoplanet observations. We present recent results from combined radiative transfer modeling (see Bailey et al. 2018) and high precision polarimeter observing efforts to examine how the inclusion of condensates in fitting models to these observations has benefitted their interpretation. We show the non-detection of polarized reflected light on WASP-18b excludes the presence of a small-droplet aerosol haze (Bott et al. 2018), how the dark nature the atmosphere of HD 189733b and many other hot Jupiters is supported by polarized light measurements (Bott et al. 2016, Bailey et al, 2021), and how rare, bright hot Jupiters like 51 Peg b are in some cases prime targets. We show the recent tentative (first) detection of polarized scattered from an atmosphere is best interpreted in the context of the condensates predicted by atmospheric models for the planet and how variations in the atmospheric model affect the polarized and unpolarized reflected light curves (Bailey et al. 2021). Finally we show predictions for hot Jupiter scenarios in polarized light to provide a means both for target selection and to aid the broader community in confirming condensation and global climate models.