Dark skies of the slightly eccentric WASP-18 b

We fit jointly more than 250 eclipse events and derive atmospheric properties using General Circulation Models and retrieval analyses. We obtain new ephemerides with unprecedented precisions of 1 second and 1.4 millisecond on the time of inferior conjunction and orbital period, respectively. We compute a planetary radius of R_p_=1.1926+/-0.0077R_J_ with a precision of 0.65%, or 550km.

Through a timing inconsistency with JWST, we discuss and confirm orbital eccentricity (e=0.00852+/-0.00091), and constrain the argument of periastron to {omega}=261.9_-1.4_^+1.3^deg. We show that the large dayside emission implies the presence of magnetic drag and super-solar metallicity. We find a steep thermally-inverted gradient in the planetary atmosphere, which is common for ultra-hot Jupiters.

We detect the presence of strong CO emission lines at 4.5{mu}m from an excess of dayside brightness in the Spitzer/IRAC/Channel 2 passband. Using these models to constrain the reflected contribution in the CHEOPS passband, we derive an extremely low geometric albedo A_CHEOPS_=0.027+/-0.011. The orbital eccentricity remains a potential challenge for planetary dynamics that might require further study given the short-period massive planet and despite the young age of the system.

The characterisation of the atmosphere of WASP-18 b reveals the necessity to account for magnetic friction and super-solar metallicity to explain the full picture of the dayside emission. We find the planetary dayside to be extremely unreflective, but when juxtaposing TESS and CHEOPS data, we get hints of increased scattering efficiency in the visible, likely due to Rayleigh scattering.