Nicholas Borsato

Sorting a Hot Mess: Decoding the 3-Dimensional Atmosphere of KELT-9b Using the Entire Visual Spectrum

The properties of ultra-hot Jupiters are so extreme that they push the defining boundaries of what constitutes an exoplanet. Among these exoplanet outliers, KELT-9b is conspicuous as the most extreme: a tidally locked, heavily irradiated, evaporating exoplanet with the highest equilibrium temperature on record (~4000 K) – and an atmosphere highly suitable for transit studies. In this study, we combined 13 transit observations from six observing sites to search for atomic and ionised species in KELT-9b’s atmosphere over a wavelength range of 316–950 nm. Utilising the cross-correlation function (CCF) as a forward model, we employed Markov Chain Monte Carlo (MCMC) sampling paired with bootstrapping techniques; bootstrapping, which involves repeatedly resampling data to estimate variability, allowed us to robustly fit velocity parameters for each detection. We aggregated all 13 nights of observations by constructing weights based on data quality, expected line depths, and model injection. Our comprehensive analysis resulted in the detection of 40 atomic and ionised species in KELT-9b’s atmosphere. To interpret these detections, we built toy models to test the hypothesis that the species originate from different locations within the atmosphere, extending even to the planet’s Roche lobe. We argue that KELT-9b’s transmission spectrum exhibits properties attributable to an inflated dayside, with an extended atmosphere partially decoupled from the planet’s interior atmosphere. Our results demonstrate that multiple cross-correlation detections can provide valuable insights into the atmospheric dynamics and geometrical properties of ultra-hot Jupiters.