When You Wish Upon A Planet: The Encapsulating Tail of KELT-9b

Irradiated Jovian atmospheres are complex, dynamic, and can undergo temporal variations that are closely influenced by their parent stars. Of the Jovian planets that have been catalogued to date, KELT-9b is the hottest known, with an equilibrium temperature of 4050 K. We probe the temporal variability of transmission spectroscopic signatures from KELT-9b via a set of archival multi-year ground-based transit observations. Our observations confirm atomic detections of H-alpha, Fe, Fe+ and Mg over multiple epochs, and that the H-alpha transit light curve deviates from a standard transit in shape. The H-alpha transit can be well described via a “cometary” tail that streams away from the planet in the direction of the observer, perpendicular to the planetary orbit. We discuss the temporal stability of this evaporative tail, and its implications on modelling the dynamics of the KELT-9b atmosphere. These multi-epoch observations, performed with the TRES facility on the 1.5 m reflector at FLWO, highlight the capabilities of small telescopes to provide temporal monitoring of the dynamics of exoplanet atmospheres.