"A Story of Condensation in Giant Exoplanet Atmospheres"

Abstract:

We understand chemistry. I dare say we understand chemistry very well. Our combined knowledge of chemistry and the natural relative distribution of elements in the universe has allowed us to predict with excellent accuracy what we should expect to see when observing atmospheres of hot giant exoplanets based primarily on their temperature. Which is why when our chemistry models predict we should see something, but we do not, it is intriguing, even surprising. Nowhere has this been so glaring as the case of titanium (Ti), which has long been theorized to drive thermal inversions in the atmospheres of giant exoplanets via the dominant optical opacity of its oxidised form, TiO.
Lo and behold, exoplanets with thermal inversions have been found aplenty but unambiguous Ti/TiO detections on these remain comparatively sparse. Moreover, abundance inferences have revealed severe levels of titanium depletion in certain planets compared to predictions, likely indicative of cold-trapping. And yet, cold-traps can be broken, as evident by the dazzling spectral presence of Ti-bearing species strongly seen on the hottest exoplanets. As a powerful absorber, the release of titanium to the gas phase introduces a sharp change in atmospheric chemistry and climate occurring over a relatively small change in temperature, analogous to the brown dwarf L to T spectral transition.
I will present recent advancements regarding the titanium situation on hot and ultra-hot giant exoplanets by highlighting new results from both JWST/NIRISS and ground-based high resolution spectrographs. Interestingly, these show that the onset of titanium in ultra-hot Jupiter atmospheres 1) matches chemical equilibrium predictions based on the nightside rather than the local temperature, 2) is delayed compared to other rock-forming metals such as iron, and 3) notably does not coincide with the onset of thermal inversions in ultra-hot Jupiters. These results not only raise doubts regarding the long held hypothesis that TiO is the primary driver of thermal inversions in highly irradiated gas giants, but also showcase the critical role that nightside cold-trapping can have in altering the predicted chemistry of exoplanet atmospheres, as well as the catastrophic biases in retrieved parameters that are induced if not properly accounted for in models.