PEPSI has detected the optical Mg I triplet at 7.8-sigma in the extended atmosphere of the ultra-hot Jupiter KELT-9 b. Constraints are placed on the density and radial extent of the excited hydrogen envelope.
Average line profiles for the Mg triplet lines of the Kelt-9 b atmosphere. ST is the ratio stellar spectrum in transit divided by stellar spectrum out of transit. Possible contaminating atomic transitions are marked with vertical red lines. Spectral resolution R is 50,000.
