Diurnal variations in the stratosphere of the ultrahot giant exoplanet WASP-121b

The temperature profile of a planetary atmosphere is key diagnostic radiative and dynamical processes governing the absorption, redistribution, emission energy. Observations have revealed dayside stratospheres that either cool or warm with altitude for small number gas giant exoplanets, while other are consistent constant temperatures. Here we report spectroscopic phase curve measurements WASP-121b, which constrain stratospheric temperatures throughout diurnal cycle. Variations measured water vapour spectral feature reveal transitions from warming on hemisphere to cooling nightside hemisphere. data well explained by models assuming chemical equilibrium, molecules thermally dissociating at low pressures recombining nightside. Nightside enough perovskite (CaTiO3) condense, could deplete titanium explain recent non-detections day-night terminator. also condensation refractory species such as magnesium, iron, vanadium. Detections these metals terminator suggest, however, if they do form clouds, cold trapping does not efficiently remove them upper atmosphere. Horizontal winds vertical mixing keep condensates aloft in until recirculated hotter vaporised.