2026AJ....171..122B

We present high-resolution near-ultraviolet observations from the Hubble Space Telescope's Space Telescope Imaging Spectrograph (STIS) data for the hottest known gas planet, KELT-9b. Observations were collected with STIS/E230M (2300-3000 Å, R ∼ 30,000), and we decorrelate systematic effects from the telescope using jitter detrending. We show the clear presence of the Mg II doublet at 2800 Å and Fe II at 2600 Å in KELT-9b. The Mg II is measured above the planet's Roche transit radius, indicating it is escaping. We fit 1D non-local thermodynamic equilibrium (NLTE) atmospheric escape models to these features, demonstrating a significant loss of mass in KELT-9b's atmosphere ($\dot{M}\approx 1{0}^{12}\,$ g s^–1); we also find a remarkably high line-broadening corresponding to a velocity of about 50-75 km s^–1, and a net blueshift of the Mg II doublet greater than 30 km s^–1. Future 3D MHD modeling of the spectrum and gas kinematics is likely needed to explain these observations. We interpret these results in the context of the Mg II “Cosmic Shoreline” and show that the detection of escaping Mg II in KELT-9b and the nondetection in WASP-178b are consistent with the hypothesis that stars hotter than T_eff ∼ 8250 K have relatively low levels of X-ray and extreme ultraviolet (XUV) radiation due to the lack of a chromosphere. Therefore, planets around such early-type stars experience a different degree of atmospheric escape. This result highlights the importance of XUV irradiation in driving atmospheric escape inside and outside the solar system.