2025MNRAS.538.1853R

A planet's albedo is a fundamental property that sets its energy budget by dictating the fraction of incident radiation absorbed versus reflected back to space. Generally, optical eclipse observations have revealed the majority of hot, giant planets to have low albedos, indicating dayside atmospheres dominated by absorption instead of reflection. However, there are several exceptions to this rule, including the ultra-hot-Neptune LTT 9779 b, which have been found to have high geometric albedos. We observed four eclipses of LTT 9779 b with the G280 grism of the Hubble Space Telescope's Wide Field Camera 3 UVIS mode; targeting the scattering signatures of the cloud condensate species causing the planet's elevated reflectivity. However, we do not definitively detect the planet's eclipse in our observations, with injection-recovery tests yielding a 3$\sigma$ upper limit of 113 ppm on the eclipse depth of LTT 9779 b in the 0.2-0.8 $\mathrm{\mu }$m waveband. We create reflectance spectrum grids for LTT 9779 b's dayside using VIRGA/PICASO and compare to our UVIS limit, as well as previously published Characterizing Exoplanets Satellite and Transiting Exoplanet Survey Satellite eclipse photometry. We find that silicate condensates are best able to explain LTT 9779 b's highly reflective dayside. Our forward model grids only enable weak constraints on vertical mixing efficiency, and suggest that, regardless of their particular composition, the clouds are likely composed of smaller and more reflective particles. Our work facilitates a deeper understanding of the reflectance properties of LTT 9779 b as well as the UVIS spectroscopic mode itself, which will remain the community's primary access to UV wavelengths until next-generation telescopes like the Habitable Worlds Observatory.