WASP-18 is a Sun-like F-type star (about 1.29 solar masses, 6,432 K, 1.32 solar radii), 123 light-years from Earth. WASP-18 b is a massive hot Jupiter, about ten Jupiter masses, and a prime target for measuring tidal orbital decay. WASP-18 b is a 10.2-Jupiter-mass companion, with an orbital period of 22.6 hours, at 0.020 AU from its star, and an equilibrium temperature near 2,429 K, discovered in 2009 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.