HD 209458 b was the first exoplanet observed transiting across the face of its star. Astronomers detected the transit in 1999, when the planet passed between Earth and its host star and briefly reduced the star’s brightness.
The planet had already been discovered through measurements of its star’s back-and-forth motion, called the radial-velocity method. Follow-up photometry by teams led by David Charbonneau and Gregory W. Henry revealed a roughly 1.7% dip in starlight. This combination of a known planet and a visible transit opened a new era of exoplanet research.
HD 209458 b is a hot Jupiter: a large gas giant orbiting extremely close to its star. It completes an orbit in about 3.52 days and is tidally locked, so the same hemisphere probably faces the star. Transit observations also allowed astronomers to measure its radius and study its atmosphere.
A frequent mix-up is OGLE-TR-56 b, which was the first planet discovered through a dedicated transit survey. HD 209458 b, however, is generally recognized as the first known transiting exoplanet observed after its radial-velocity discovery.