HAT-P-21 is a Sun-like G-type star (about 1.24 solar masses, 5,588 K, 1.21 solar radii), 277 light-years from Earth. HAT-P-21 b is a 4.87-Jupiter-mass companion, with an orbital period of 4.1 days, on an eccentric orbit (e ≈ 0.23), discovered in 2010 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.