2022ApJ...937...90D

Relatively little is understood about the atmospheric composition of temperate to warm exoplanets (equilibrium temperature T_eq < 1000 K), as many of them are found to have uncharacteristically flat transmission spectra. Their flattened spectra are likely due to atmospheric opacity sources such as planet-wide photochemical hazes and condensation clouds. We compile the transmission spectra of 25 warm exoplanets previously observed by the Hubble Space Telescope and quantify the haziness of each exoplanet using a normalized amplitude of the water absorption feature (A_H). By examining the relationships between A_H and various planetary and stellar forcing parameters, we endeavor to find correlations of haziness associated with planetary properties. We adopt new statistical correlation tests that are more suitable for the small, nonnormally distributed warm exoplanet sample. Our analysis shows that none of the parameters have a statistically significant correlation with A_H (p ≤ 0.01) with the addition of new exoplanet data, including the previously identified linear trends between A_H and T_eq or the hydrogen-helium envelope mass fraction (f_HHe). This suggests that haziness in warm exoplanets is not simply controlled by any single planetary/stellar parameter. Among all the parameters we investigated, planet gravity (g_p), atmospheric scale height (H), planet density (ρ_p), orbital eccentricity (e), and age of the star (t_age) have tentative correlations with A_H. Specifically, lower H, higher g_p, ρ_p, e, or t_age may lead to clearer atmospheres. We still need more observations and laboratory experiments to fully understand the complex physics and chemistry involved in creating hazy warm exoplanets.