A simultaneous reflected-light and thermal emission spectroscopic phase curve of an exo-Neptune

Louis Philippe Coulombe ( Université de Montréal )

Hot Neptune exoplanets defy our understanding of atmosphere evolution, as such objects should not be able to retain a substantial atmosphere against the extreme irradiation received from their host star. The exoplanet LTT 9779 b is the most irradiated Neptune-sized planet known to date, making it the ideal target to study the chemistry and dynamics of an ultra-hot exoplanet in the lower mass regime, which has yet to be explored. We present the spectroscopic phase curve and phase-resolved emission spectra of the ultra-hot Neptune LTT 9779b obtained with JWST NIRISS/SOSS. The observations simultaneously cover the reflected light and thermal emission-dominated regimes of the planetary spectrum at all orbital phases. We detect a distinct difference in the phase curve offsets in those two regimes, with the maximum in the reflected light indicating a highly reflective western dayside, while the thermal emission is at maximum near the substellar point with a slight asymmetry to the east. The retrieved albedos and temperature-pressure profiles across orbital phases are both well explained by a global circulation and cloud pattern involving silicate clouds being transported from the cold nightside westwards towards the dayside hot spot by an equatorial jet, where they then evaporate near the hottest longitudes of the planet before recondensing towards the eastern terminator. The transport of these clouds towards the dayside, which block an important amount of the irradiation coming from its host star, could explain the survival of LTT 9779b in the hot Neptune desert.