Titanium oxide and chemical inhomogeneity in the atmosphere of the exoplanet WASP-189b

Ultra-hot Jupiters are a class of highly irradiated planets that have fundamentally changed our view of planetary systems. Located in very short orbits around their host stars, ultra-hot Jupiters are subject to extreme conditions with permanently irradiated hot day sides and cooler, permanently dark night sides. The hot temperature regime causes a variety of different effects, such as the thermal dissociation of many molecules, partial thermal ionisation of atoms, and the occurrence of thermal inversions caused by metals and their oxides. The temperature of an atmosphere decreases with increasing altitude unless there is a shortwave absorber that causes a temperature inversion. While this absorber is known to be ozone in Earth's atmosphere, titanium oxide and vanadium oxide are predicted to be shortwave observers in the atmospheres of ultra-hot Jupiters. Using ground-based, high-resolution spectroscopy, we have been able to study the atmosphere of WASP-189 b, an ultra-hot Jupiter with an equilibrium temperature of 2600 K. By applying the cross-correlation technique to five independent observations, we have detected titanium oxide, as well as various different metals, including neutral and singly ionised iron and titanium, as well as chromium, magnesium, vanadium and manganese. Our detections reveal the presence of dynamical effects that show up in deviations from the true orbital and systemic velocities. We interpret these as a consequence of spatial gradients in their chemical abundances, suggesting their existence in different regions or dynamical regimes. Our findings are direct observational evidence for the 3D thermochemical stratification of an exoplanet atmosphere derived from high-resolution ground-based spectroscopy. As more and more ultra-hot Jupiters are observed, our observations show empirically that successful interpretation of observations of this type of planet requires that the theory of exoplanet atmospheres accounts for the 3D nature of these atmospheres and that insights from global circulation models, atmospheric chemistry and radiative transfer are unified