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 Sub-Neptune Planetary Atmosphere Characterization Experiment (SPACE) 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 and 1.7 μ 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 σ and 3.2 σ against a cloud-free atmosphere with solar metallicity, rendering this scenario unlikely, but still possible. A supersolar 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 ± 20) K, particularly in atmospheres with supersolar metallicity, possibly additionally muting absorption features.