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The atmospheric temperatures of the ultra-hot Jupiter KELT-9b straddle thetransition between gas giants and stars, and therefore between twotraditionally distinct regimes of atmospheric chemistry. Previous theoreticalstudies assume the atmosphere of KELT-9b to be in chemical equilibrium. Despitethe high ultraviolet flux from KELT-9, we show using photochemical kineticscalculations that the observable atmosphere of KELT-9b is predicted to be closeto chemical equilibrium, which greatly simplifies any theoreticalinterpretation of its spectra. It also makes the atmosphere of KELT-9b, whichis expected to be cloudfree, a tightly constrained chemical system that lendsitself to a clean set of theoretical predictions. Due to the lower pressuresprobed in transmission (compared to emission) spectroscopy, we predict theabundance of water to vary by several orders of magnitude across theatmospheric limb depending on temperature, which makes water a sensitivethermometer. Carbon monoxide is predicted to be the dominant molecule under awide range of scenarios, rendering it a robust diagnostic of the metallicitywhen analyzed in tandem with water. All of the other usual suspects (acetylene,ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to besubdominant at solar metallicity, while atomic oxygen, iron and magnesium arepredicted to have relative abundances as high as 1 part in 10,000. Neutralatomic iron is predicted to be seen through a forest of optical andnear-infrared lines, which makes KELT-9b suitable for high-resolutionground-based spectroscopy with HARPS-N or CARMENES. We summarize futureobservational prospects of characterizing the atmosphere of KELT-9b.

The Peculiar Atmospheric Chemistry of KELT-9b