Quantifying and Predicting the Presence of Clouds in Exoplanet Atmospheres

Quantifying and Predicting the Presence of Clouds in Exoplanet Atmospheres

Authors:

Stevenson et al

Abstract:

One of the most outstanding issues in exoplanet characterization is understanding the prevalence of obscuring clouds and hazes in their atmospheres. The ability to predict the presence of clouds/hazes a priori is an important goal when faced with limited telescope resources and advancements in atmospheric characterization that rely on the detection of spectroscopic features. As a means to identify favorable targets for future studies with HST and JWST, we use published HST/WFC3 transmission spectra to determine the strength of each planet's water feature, as defined by the H2O - J index. By expressing this parameter in units of atmospheric scale height, we provide a means to efficiently compare the size of spectral features over a physically diverse sample of exoplanets. We find the H2O - J index to be strongly correlated with planet temperature when Teq less than 750+90−60 K and weakly correlated with surface gravity for planets with logg less than 3.2+0.3−0.2 dex. Otherwise, the median value of the H2O - J index is 1.8±0.3 H. Using these two physical parameters, we identify a division between "classes" of exoplanets, such that objects above Teq=700 K and logg=2.8 dex are more likely to have clearer atmospheres with stronger spectral features (H2O - J greater than 1) and those below at least one of these thresholds are increasingly likely to have predominantly cloudy atmospheres with muted spectral features (H2O - J less than 1). Additional high-precision measurements are needed to corroborate the reported trends.