HAT-P-31 is a Sun-like F-type star (about 1.31 solar masses, 6,065 K, 1.39 solar radii), 344 light-years from Earth. HAT-P-31 b is a 2.27-Jupiter-mass companion, with an orbital period of 5.0 days, on an eccentric orbit (e ≈ 0.25), discovered in 2011 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 marked eccentricity suggests past scattering events or perturbations from unseen companions.