| Observation data|
|Right ascension||22h 03m 10.8s|
|Declination||+18° 53′ 04″|
|Apparent magnitude (V)||7.65|
|Spectral type||F8-G0 V|
|B-V color index||0.59|
|U-B color index|
|Variable type||Planetary transit|
| Astrometry <tr valign=top><td>Radial velocity (Rv)</td><td>-14.8 km/s</td></tr><tr valign=top><td>Proper motion (μ)</td><td> RA: 28.90 mas/yr |
Dec.: -18.37 mas/yr </td></tr><tr valign=top><td>Parallax (π)</td><td>21.24 ± 1.00 mas</td></tr><tr valign=top><td>Distance</td><td>154 ly (47.1 pc)</td></tr><tr valign=top><td>Absolute magnitude (MV)</td><td>4.29</td></tr>
|Age||4-7 × 109 years|
HD 209458 is an 8th magnitude star in the constellation Pegasus. It is very similar to our Sun, and it is classified as a yellow dwarf (spectral class G0 V). Because it is located at a distance of about 150 light years, it is not visible to the unaided eye. With good binoculars or small telescope it should be easily detectable.
In 1999 two teams working independently (one team consisted of astronomers at the Geneva Observatory, the Harvard-Smithsonian Center for Astrophysics, and the Wise Observatory; the second group was the California and Carnegie Planet Search team) discovered an extrasolar planet orbiting the star by using the radial velocity planet search method. Soon after the discovery, separate teams led by David Charbonneau and Gregory W. Henry were able to detect a transit of the planet across the surface of the star making it the first known transiting extrasolar planet. The planet received the designation HD 209458 b.
Spectroscopic studies first revealed the presence of a planet around HD 209458 on November 5 1999. Astronomers had made careful photometric measurements of several stars known to be orbited by planets, in the hope that they might observe a dip in brightness caused by the transit of the planet across the star's face. This would require the planet's orbit to be inclined such that it would pass between the Earth and the star, and previously no transits had been detected.
(In order from star)
| Orbital period|
| Semimajor axis|
|b||0.69 ± 0.05||3.52474541 ± 0.00000025||0.045||0.00|
- Tsevi Mazeh et al, 2000, "The Spectroscopic Orbit of the Planetary Companion Transiting HD 209458", Astrophysical Journal, vol. 532, pp. L55–L58.