WASP-17 is a Sun-like F-type star (about 2.28 solar masses, 6,550 K, 1.49 solar radii), 406 light-years from Earth. WASP-17 b was the first exoplanet shown to orbit retrograde, opposite its star's spin, and it is one of the most inflated hot Jupiters. WASP-17 b is a 0.78-Jupiter-mass companion, with an orbital period of 3.7 days, 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. The planet's large radius (21.0 Earth radii) indicates a bloated atmosphere heated by intense stellar radiation.