Near-Ultraviolet Transmission Spectroscopy of the Ultra-hot Jupiter KELT-9b from the Colorado Ultraviolet Transit Experiment

KELT-9b is the hottest discovered exoplanet to date and ground-based transmission spectroscopy of this ultra-hot Jupiter has revealed a wealth of atoms and ions entrapped in its escaping atmosphere. Here we present new near-ultraviolet (NUV) observations of the planet obtained with the Colorado Ultraviolet Transit Experiment (CUTE) CubeSat. The CUTE transmission spectra show regions of extended absorption at altitudes beyond the planet’s Roche lobe, including a spectral region containing Mg II. We present and explore the KELT-9b NUV transmission spectrum and place it in the context of previous observations.