XO-1 is a Sun-like G-type star (about 0.88 solar masses, 5,750 K, 0.88 solar radii), 164 light-years from Earth. XO-1 b is a 0.83-Jupiter-mass companion, with an orbital period of 3.9 days, discovered in 2006 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 (12.8 Earth radii) indicates a bloated atmosphere heated by intense stellar radiation.