Deciphering Cloud Properties for the Warm Neptune HAT-P-11b Using Optical Transmission Spectroscopy
Transmission spectroscopy offers a powerful window into exoplanetary atmospheres, but a majority of the systems observed to date appear to have high altitude cloud layers that attenuate the expected atmospheric absorption signal. This is particularly problematic for smaller and cooler planets, which pose a significant challenge for atmospheric characterisation efforts. We present new HST STIS optical transmission spectroscopy for the warm Neptune HAT-P-11b in the 300-900 nm wavelength range, and re-analyse previously published infrared observations taken by HST WFC3 and Spitzer IRAC. We detect a significant slope in the optical bandpass due to scattering from cloud particles, and use these observations to constrain the properties of this cloud layer. We compare the results of these retrievals to the predictions of microphysical cloud models and discuss the corresponding implications for our understanding of cloud formation processes in these atmospheres. Finally, we place the atmospheric properties of HAT-P-11b in context with those of other comparably small and cool planets.