Quantifying and Predicting the Presence of Clouds in Exoplanet Atmospheres

TL;DR

This study uses HST/WFC3 spectra to analyze water features in exoplanet atmospheres, identifying physical parameters that predict cloud presence and atmospheric clarity, aiding future observational targeting.

Contribution

It introduces the H2O - J index as a comparative measure of spectral feature strength and correlates it with planet temperature and gravity to predict cloudiness.

Findings

H2O - J index correlates with temperature below 750 K.

Planets above 700 K and gravity above 2.8 dex tend to have clearer atmospheres.

Thresholds in temperature and gravity divide planets into cloudy and clear atmospheric classes.

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 $T_{eq} < 750^{+90}_{-60}$ K and weakly correlated with surface gravity for planets with $\log g < 3.2^{+0.3}_{-0.2}$ dex. Otherwise, the median value of the H2O - J index is $1.8{\pm}0.3$ 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.