No TiO detected in the hot-Neptune-desert planet LTT-9779 b in reflected light at high spectral resolution

LTT-9779 b is an inhabitant of the hot-Neptune desert and one of only a few planets with a measured high albedo. Characterising the atmosphere of this world is the key to understanding the processes that dominate in reducing the number of short-period intermediatemass planets that create the hot-Neptune desert. We aim to characterise the reflected light of LTT-9779 b at high spectral resolution to break the degeneracy between clouds and atmospheric metallicity. This is key to interpreting its mass-loss history, which might illuminate how it kept its place in the desert. We used the high-resolution cross-correlation spectroscopy technique on four half-nights of ESPRESSO observations in 4-UT mode (16.4 m effective mirror) to constrain the reflected-light spectrum of LTT-9779 b. We did not detect the reflected-light spectrum of LTT-9779 b although these data had the expected sensitivity at the level 100 ppm. Injection tests of the post-eclipse data indicated that TiO should have been detected for a range of different equilibrium chemistry models. Therefore, this non-detection suggests TiO depletion in the western hemisphere, but this conclusion is sensitive to temperature, which affects the chemistry in the upper atmosphere and the reliability of the line list. Additionally, we were able to constrain the top of the western cloud deck to Ptop, western < 10−2∙0 bar and the top of the eastern cloud deck to Ptop, eastern < 10−0∙5 bar, which is consistent with the predicted altitude of MgSiO3 and Mg2SiO4 clouds from JWST NIRISS/SOSS. While we did not detect the reflected-light spectrum of LTT-9779 b, we verified that this technique can be used in practice to characterise the reflected light of exoplanets at high spectral resolution when their spectra contain a sufficient number of deep spectral lines. Therefore, this technique may become an important cornerstone of exoplanet characterisation with the ELT and beyond.