Presentation #102.322 in the session Poster Session.
Relatively little is understood about the atmospheric composition of warm exoplanets (equilibrium temperature Teq < 1000 K), as many of them are found to have uncharacteristically flat spectra (e.g., ). Their flattened spectra are likely due to opacity sources such as planet-wide photochemical hazes and condensation clouds in their atmospheres (see [2,3] and references within). To quantify the haziness/cloudiness of an exoplanet, we compile and compare the transmission spectra of 23 temperate exoplanets previously observed by the Hubble Space Telescope (HST). By examining the relationship between the derived normalized transit radius difference from the water absorption feature’s amplitude (AH) and various planetary/stellar parameters, we endeavor to find correlations of planetary properties with haziness/cloudiness. In addition to the previously used Pearson’s Correlation Coefficient, we also adopt Spearman’s Rho and Kendall’s Tau statistical tests that are more suitable for a small sample showing non-parametric distribution of data, like our current exoplanet sample. Our analysis shows that the previously established linear trends  between AH and Teq/hydrogen-helium envelope fraction (fHHe) break down with the addition of new exoplanets. Among the 12 planetary (mass, radius, density (⍴p), gravity (gp), semi-major axis, eccentricity (e), Teq, atmospheric scale height (H), fHHe, XUV/FUV/NUV fluxes) and 9 stellar (mass, radius, density, gravity, age, effective temperature, rotation period, metallicity, luminosity) parameters we investigated, none hold statistically significant correlations with AH (p ⩽ 0.01) using the Spearman’s Rho and Kendall’s Tau tests. Tentatively, we find possible correlations (p ⩽ 0.06) with gp, ⍴p, H, and e. Specifically, higher gp, ⍴p, e, or lower H leads to clearer atmospheres. This suggests that haziness in warm exoplanets is not dominantly controlled by a single planetary/stellar parameter. More observations and laboratory experiments are needed to fully understand the complex physical and chemical processes that lead to the formation and removal of photochemical hazes and condensation clouds in warm exoplanet atmospheres.
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