Reflected light and dayside emission from the phase curves of the ultra-hot Jupiter WASP-18 b

The extra-solar planet WASP-18 b is placed on a very-short period (< 1 day) orbit around a F6V-type star (T_eff = 6300 K), which results in a planetary dayside heated up to 3000 K. Planets with such extreme thermal conditions are called ultra-hot Jupiters (UHJs) and exhibit strong day-to-night temperature contrasts with atmospheric compositions dominated by dissociated molecules and ions. These properties make UHJs such as WASP-18 b particularly interesting for atmospheric characterisation all along their orbital phase angles through observations known as phase curves.

We present our work based on the analysis of phase-curve and occultation observations with the space telescopes CHEOPS, TESS and Spitzer, including unpublished data from all three instruments. The data sets span a temporal range of 15 years and a spectral range from the visible to the mid-infrared. We model and constrain the ellipsoidal variations and Doppler boosting, obtain unprecedented precision on the ephemerides, and derive an absolute planetary radius with a precision of 0.65%, or 550 km. We include the recently published JWST data (Coulombe et al. 2023) in our atmospheric retrieval, from which we derive constraints on the structure and composition of the atmosphere of WASP-18 b. We find a thermally inverted profile with a steep temperature gradient to explain the brightness temperature in Spitzer/IRAC/Channel 2 passband. We also detect a flux excess in the CHEOPS passband (visible) that allows us to assess the presence of reflected light and compute the first constained value of the geometric albedo for this planet.