2016ApJ...817L..16S

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 Hubble Space Telescope (HST) and James Webb Space Telescope, we use published HST/WFC3 transmission spectra to determine the strength of each planet's water feature, as defined by the H₂ O - 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 T_eq < 750_–60⁺⁹⁰ K and weakly correlated with surface gravity for planets with log g < 3.2_–0.2^+0.3 dex. Otherwise, the median value of the H₂ O - 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 T_eq= 700 K and log g 2.8 dex are more likely to have clearer atmospheres with stronger spectral features (H₂O - J > 1) and those below at least one of these thresholds are increasingly likely to have predominantly cloudy atmospheres with muted spectral features (H₂O - J < 1). Additional high-precision measurements are needed to corroborate the reported trends.