2025A&A...695A..26R

Context. The proliferation of exoplanet discoveries, particularly within such exotic environments as the Neptune desert, challenges our understanding of planetary atmospheres undergoing intense irradiation. The unexpected discovery of LTT 9779 b, an ultra-hot Neptune deep within this desert offers a prime opportunity for in-depth atmospheric studies. This research builds upon previous observations of LTT9779b from space-based telescopes, including the Transiting Exoplanet Survey Satellite (TESS), Spitzer Space Telescope, and CHaracterising ExOPlanet Satellite (CHEOPS), while incorporating new observations from the Very Large Telescope's (VLT) Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO) instrument to delve deeper into the atmospheric dynamics of this intriguing exoplanet. Preliminary analyses suggest a metal-rich atmosphere alongside a notably high day-side geometric albedo that may imply the existence of silicate clouds. Furthermore, there appears to be minimal atmospheric escape, presenting intriguing contrasts to existing models of planetary evolution and atmospheric behaviour under extreme irradiation.
Aims. We aim to contribute to the broader understanding of atmospheric compositions and the mechanisms behind the survival of atmospheres in the Neptune desert through detailed spectroscopic analysis. We started by obtaining the transmission spectrum of LTT9779 b between 0.4 and 0.78 micrometres with ESPRESSO on the VLT.
Methods. Our analysis addressed systematics in ESPRESSO data across three distinct transit events, focusing on the sodium doublet and hydrogen alpha (Hα). We also used the cross-correlation method with models that contain Na, K, FeH, TiO, and VO
Results. No statistically significant atmospheric signal was detected, with lower limits placed on the atmospheric metallicity established at [Fe/H] ≥ 2.25, which is ≥ 180× solar. The non-detection is aligned with a high metallicity atmosphere scenario in a cloud-free model, suggesting a high mean molecular weight and a reduced atmospheric scale height.
Conclusions. We interpret the lack of any detection as evidence to support a very high metallicity for the planet's atmosphere. This would give rise to a high mean molecular weight and, hence, a low atmospheric scale height, rendering any signal too weak to be detected. Another possibility is the presence of high-altitude clouds or hazes that would suppress any signal from elements deeper in the atmosphere. These findings are consistent with recent consistent with recent James Webb Space Telescope (JWST) observations, which also report muted spectral features and suggest a high-metallicity atmosphere with clouds at high altitudes. Our results, together with those from JWST, support the hypothesis of a metal-rich atmosphere possibly obscured by clouds or hazes.