Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission

TL;DR

This study combines multi-wavelength phase-curve data from CHEOPS, TESS, and Spitzer to analyze the atmosphere and orbital parameters of ultra-hot Jupiter WASP-18 b, revealing a steep thermal inversion, magnetic effects, and low reflectivity.

Contribution

It provides the first comprehensive multi-wavelength analysis of WASP-18 b's atmosphere and orbit, including new precise ephemerides and insights into atmospheric composition and dynamics.

Findings

Detected strong CO emission lines at 4.5 μm.

Constrained orbital eccentricity to e=0.00852.

Derived a very low geometric albedo of 0.027.

Abstract

We performed a joint analysis of phase-curve observations of the ultra-hot Jupiter WASP-18 b from the visible to the mid-infrared, using data from CHEOPS, TESS and Spitzer. We aim to characterise the planetary atmosphere with a consistent view over the large wavelength range covered using GCMs and retrieval analyses, and including JWST data. We obtained new ephemerides with unprecedented precisions of 1 second and 1.4 millisecond on the time of inferior conjunction and orbital period, respectively. We computed a planetary radius of $R_p = 1.1926 \pm 0.0077 R_J$ with a precision of 0.65% (or 550 km). Based on a timing inconsistency with JWST, we discuss and confirm orbital eccentricity ($e = 0.00852 \pm 0.00091$). We also constrain the argument of periastron to $\omega = 261.9^{+1.3}_{-1.4}$ 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 UHJs. We detected the presence of strong CO emission lines at 4.5 $\mu$m from an excess of dayside brightness in the Spitzer/IRAC/Ch2 passband. Using these models to constrain the reflected contribution in the CHEOPS passband, we derived an extremely low geometric albedo of $A_g^\text{CHEOPS} = 0.027 \pm 0.011$.