HD 209458 is a Sun-like F-type star (about 1.23 solar masses, 6,091 K, 1.19 solar radii), 48.3 light-years from Earth. HD 209458 b was the first exoplanet detected by transit and the first to have its atmosphere studied spectroscopically, establishing it as a benchmark for exoplanet characterisation. HD 209458 b is a 0.73-Jupiter-mass companion, with an orbital period of 3.5 days, discovered in 1999 via radial velocity. The planet's large radius (15.6 Earth radii) indicates a bloated atmosphere heated by intense stellar radiation. Long-term spectroscopic monitoring of HD 209458 continues to refine the planet's mass and search for additional companions.