The SPACE Program II: No discernible spectral features in the transmission spectrum of the sub-Neptune HD 191939 b observed with HST/WFC3

The atmospheres of sub-Neptunes provide a window into their internal structure and history, shedding light on the origin of this common, but enigmatic, class of exoplanets. However, the physical and chemical processes that shape sub-Neptunes' transmission spectra, in particular cloud and haze formation, are not well understood. To identify possible correlations between transmission spectra and UV irradiation, the SPACE (Sub-neptune Planetary Atmosphere Characterization Experiment) Program observed an array of sub-Neptunes and their host stars using the Hubble Space Telescope (HST), measuring the planets' transmission spectra between 1.1, mu m and 1.7, mu m with the Wide Field Camera 3 (WFC3) and the stars' UV spectra with the Space Telescope Imaging Spectrograph (STIS). Here, we present the observations of HD 191939 b carried out as part of the SPACE Program, which reveal no significant spectral features in the transmission spectrum. The data deliver moderate evidence at significance levels between 2.0, sigma and 3.2, sigma against a cloud-free atmosphere with solar metallicity, rendering this scenario unlikely, but still possible. A super-solar metallicity of HD 191939 b might be consistent with the known trend of increasing atmospheric metallicity with decreasing planet mass. Both hydrocarbon haze formation and cloud condensation can be efficient at HD 191939 b's zero-albedo equilibrium temperature of (880 plus or minus 20), K, particularly in atmospheres with super-solar metallicity, possibly additionally muting absorption features.