Cool, Cloudy Nights on an Ultrahot Neptune: A Panchromatic NIRISS/NIRSpec Transmission Spectrum of LTT 9779 b

Studies of planets that reside in the hot Neptune desert allow us to gain insight into how their atmospheres survive the intense flux from their host stars. One such member of that population, LTT 9779 b, is unique in that it has a confirmed atmosphere across various multi-wavelength studies, although results regarding the planet's atmospheric content differ. We present a panchromatic (λ= 0.63 - 5.165 μ m) JWST transmission spectrum of LTT 9779 b's atmosphere, including a re-reduction of previously obtained NIRISS/SOSS data and a reduction of recent NIRSpec/G395H data. Through atmospheric retrievals, we find muted spectral features consistent with the presence of clouds and a low terminator temperature (about560 K for free chemistry, and about910-950 K for equilibrium chemistry), with a range of possible atmospheric metallicity values dependent on atmospheric temperature and cloud coverage. Forward modeling suggests that, if clouds are present, small sub-micron particle sizes, moderate-to-high sedimentation efficiency (f sed = 1 - 3, corresponding to a relatively compact vertical cloud extent), low particle column densities, and high eddy diffusion coefficients (k zz at least 10 10 cm squared s-1) are preferred. Probing silicate cloud absorption features in the mid-infrared is needed to confirm the cloud species, allowing us to break the cloud-metallicity degeneracy that is present in this panchromatic transmission spectrum.