The atmosphere and architecture of WASP-189 b probed by its CHEOPS phase curve

Gas giants orbiting close to hot and massive early-type stars can reach dayside temperatures that are comparable to those of the coldest stars. These “ultra-hot Jupiters” have atmospheres made of ions and dissociated molecules and feature strong day-to-night temperature gradients. Photometric observations of such planets at different orbital phases (e.g. transits, eclipses) provide insights on their atmospheric properties.

In this talk, we present the analysis of the photometric observations of WASP-189 acquired with the instrument CHEOPS, from which we derive constraints on the system architecture and the planetary atmosphere. We describe our implementation of a light curve model suited for asymmetric transit shape caused by the gravity-darkened photosphere of the fast-rotating host star. Our approach also includes modelling of the reflective and thermal components of the planetary flux, and precise timing of the transit and eclipse events by accounting for stellar oblateness and light-travel time. In addition, the model corrects for systematic noise typical for CHEOPS observations and features a Gaussian process to fit for stellar activity.

From the asymmetric transit, we measure the size of the ultra-hot Jupiter WASP-189 b, R_p = 1.600^+0.017_-0.016 R_J, with a precision of 1%, and the true orbital obliquity of the planetary system Ψ_p = 89.6 ± 1.2 deg, which is fully consistent with a polar orbit. The phase curve does not feature any significant hotspot offset (-7 ± 17 deg) and we robustly constrain its amplitude from the eclipse depth δ_ecl = 96.5^+4.5_-5.0 ppm. This value provides an upper limit on the geometric albedo of WASP-189 b: A_g < 0.48. We find that thermal emission only is marginally consistent (at 1.6 σ) with such an eclipse providing hints that the atmosphere either has extremely low Bond albedo and heat redistribution efficiency or is quite reflective. Finally, we attribute the photometric variability detected in the data to the star and its rotation, which can be explained by either superficial inhomogeneities or resonance couplings between the convective core and the radiative envelope.

Based on the derived system architecture, we predict the eclipse depth in the upcoming TESS observations to be up to ∼ 165 ppm. High-precision detection of the eclipse in both CHEOPS and TESS passbands might help disentangle between reflective and thermal contributions. We also expect the right ascension of the ascending node of the orbit to precess due to the perturbations induced by the stellar quadrupole moment J₂ (oblateness). This effect can be directly quantified by a variation of the impact parameter.