WASP-121 is a Sun-like F-type star (about 1.33 solar masses, 6,628 K, 1.46 solar radii), 270 light-years from Earth. WASP-121 b is an ultra-hot Jupiter with a detected stratosphere, and heavy elements have been seen escaping from its dayside. WASP-121 b is a 1.17-Jupiter-mass companion, with an orbital period of 1.3 days, at 0.026 AU from its star, and an equilibrium temperature near 2,409 K, discovered in 2016 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.