KELT-9 is a bright A-type star (about 2.32 solar masses, 9,270 K, 2.42 solar radii), 204 light-years from Earth. KELT-9 b is the hottest known giant planet, orbiting an A-type star so closely that dayside molecules are dissociated. KELT-9 b is a 2.88-Jupiter-mass companion, with an orbital period of 1.5 days, at 0.034 AU from its star, and an equilibrium temperature near 3,921 K, discovered in 2017 via transit. The inner world is a hot Jupiter — a gas giant hugging its star at a fraction of Mercury's distance — a class that forced astronomers to rethink how giant planets form and migrate inward.