HD 209458

HD 209458 (informally Osiris) is a sun-like G0 V star 159 light-years away in Pegasus, magnitude 7.64. In 1999 it became the first star where an exoplanet was observed transiting — HD 209458 b, a hot Jupiter that crosses the stellar disc every 3.52 days, dimming the star by 1.7 %. The detection by David Charbonneau and colleagues launched the era of transit-based exoplanet science and transmission spectroscopy.

Comfortable in any small telescope from a dark sky, or in giant binoculars. The transits themselves are detectable with amateur equipment — a 200mm telescope, a CCD, and careful comparison-star photometry can recover the 0.017-magnitude dip. Predicted transit times are listed on the Czech Astronomical Society's Exoplanet Transit Database. Located in southern Pegasus.

Charbonneau, Brown, Latham and Mayor announced the first transit detection in November 1999, three months after Henry, Marcy and collaborators independently observed the same event. The 2002 Hubble STIS detection of sodium absorption in the planet's atmosphere by Charbonneau et al. was the first identification of any chemical species in an exoplanetary atmosphere.