Bridging atmospheres and interiors: Exploring exoplanetary gas giants

With advancing observations of exoplanets, there is a pressing need for models that can explain observation trends. Detailed spectra of exoplanetary atmospheres reveal their chemical composition, while radius and mass measurements provide information about the planet's bulk composition. The composition of planets is strongly influenced by their formation process, making it useful to compare theories of planet formation with present-day observations. Hot Jupiters, gas giant planets located close to their star, always face the star with the same side. Due to their proximity and asymmetric illumination, hot Jupiters experience strong winds that shape their climate. Additionally, many of these hot Jupiters have lower densities compared to the gas giants in our solar system, suggesting an unknown mechanism that inflates these planets. Previous studies have proposed that atmospheric dynamics could deposit a fraction of the stellar irradiation deep enough in the atmosphere to cause these planets to exhibit these inflated radii.

We used planet formation models to investigate how pebble drift in protoplanetary disks affects the composition of planets that form in these disks. Using these models, we discovered that the composition of atmospheres of gas giant planets can be significantly influenced by the evaporation of pebbles in the disk. Pebbles drifting inward cross evaporation lines, causing parts of their composition to evaporate. This leads to the pollution of the gas phase of the disk with heavy elements and the enrichment of forming planet with volatile species like H2O and CO2. These insights from pebble evaporation are important for putting the carbon to oxygen ratio and possibly the volatile to refractory ratio in the atmosphere of exoplanets into the correct formation context. However, only the uppermost 1% of the planetary radius are accessible via detailed observations, whereas understanding planet formation requires an understanding of the planetary bulk composition. Thus, a better connection between the observable atmosphere and the deeper layers is a pressing need. 

We then coupled an efficient radiative transfer solver to a 3D climate model, to study the link between the deeper atmospheric layers and the radiative upper atmosphere. A long-term study showed that we can indeed realize a first link between the observable atmosphere and deeper layers for the example of WASP-76b . We find that it is possible to transport energy from the irradiated photosphere downwards, potentially explaining inflated radius. However, since this process of energy disposition is very slow, temperature convergence in deep atmospheric layers is out of reach for climate models of hot Jupiters. Future atmospheric models of hot Jupiters should thus base their initial temperature profile in the deep layers on reasonable interior structure model estimates. With a tighter link to the interior, detailed spectra from the James Webb Space Telescope, will hopefully provide a window into the formation of these planets.