HD 209458

HD 209458
Name	HD 209458
Constellation	Pegasus
Epoch	J2000
Ra	22h 03m 10.7722s
Dec	+18° 53′ 03.543″
Appmag v	7.65
Spectral type	G0V
Mass	1.148 M☉
Radius	1.203 R☉
Luminosity	1.63 L☉
Temperature	6065 K
Metallicity	+0.02 [Fe/H]
Age	3.1 Gyr
Names	BD+18°4917, HIP 108859, SAO 107741

HD 209458 is a G-type main-sequence star in the constellation Pegasus notable for hosting a transiting exoplanet that revolutionized observational exoplanetology. The system has been central to studies by teams associated with California Institute of Technology, NASA, and European Southern Observatory and has appeared in observational programs using facilities such as the Hubble Space Telescope, the Spitzer Space Telescope, and the Keck Observatory. Its discovery and follow-up have linked researchers from institutions including Harvard University, Massachusetts Institute of Technology, and University of California, Berkeley.

Overview

HD 209458 is located roughly 159 light-years from the Sun and became widely studied after the detection of a transiting exoplanet enabled precise measurements by collaborations involving the Trans-Atlantic Exoplanet Survey, the Wide Angle Search for Planets, and teams from Geneva Observatory and Observatoire de Haute-Provence. The star’s brightness and sky position have made it a target for programs at the Palomar Observatory, Calar Alto Observatory, and McDonald Observatory, while data analysis has engaged researchers affiliated with Space Telescope Science Institute, Jet Propulsion Laboratory, and the European Space Agency.

Stellar Characteristics

The host is classified as a G0V star with parameters refined through spectroscopy by groups at Keck Observatory, Very Large Telescope, and Anglo-Australian Telescope. Stellar mass and radius estimates derive from stellar evolution models developed at Steward Observatory, Instituto de Astrofísica de Canarias, and Max Planck Institute for Astronomy. Effective temperature, surface gravity, and metallicity have been compared against standards from the Geneva-Copenhagen Survey, the Hipparcos Catalogue, and analyses at Copenhagen University. Age dating techniques have used isochrones from Yonsei-Yale, Padova, and Dartmouth Stellar Evolution Program computations, with activity indicators cross-referenced to results from Mount Wilson Observatory and chromospheric surveys connected to Lowell Observatory.

Planetary System

The system’s primary planet, discovered via radial-velocity work at Observatoire de Haute-Provence and photometric transit detection programs linked to Harvard-Smithsonian Center for Astrophysics teams, was confirmed through follow-up with instruments at W. M. Keck Observatory and Lick Observatory. The planet’s orbital parameters and mass measurements were refined using methods developed at Carnegie Institution for Science, University of Geneva, and University of California, Santa Cruz. Subsequent searches for additional companions involved instruments from European Southern Observatory, Subaru Telescope, Gemini Observatory, and long-term monitoring by groups at University of Texas at Austin and University of Arizona.

Transit Observations and Discoveries

Transit photometry of the system provided one of the first opportunities to measure an exoplanetary radius, work led by teams at California Institute of Technology, Space Telescope Science Institute, and collaborators from Max Planck Institute for Astronomy. Observations with the Hubble Space Telescope’s instruments, coordinated via NASA Goddard Space Flight Center and data pipelines from STScI, enabled detection of atmospheric absorption and precise timing used by analysts at Princeton University, Yale University, and Columbia University. Ground-based campaigns by researchers at Arizona State University, University of Florida, and Pennsylvania State University complemented space-based transit timing studies, which also involved theoretical modeling from Massachusetts Institute of Technology and California Institute of Technology.

Atmospheric Studies

Spectroscopic transit and occultation studies using the Hubble Space Telescope, the Spitzer Space Telescope, and ground-based spectrographs on Keck Observatory and VLT revealed sodium absorption and atmospheric escape phenomena, with teams at University of Colorado Boulder, University of Cambridge, and University of Oxford contributing analyses. Investigations into exospheric hydrogen and carbon detections involved instrumentation and expertise from European Space Agency, Belgian Institute for Space Aeronomy, and groups at NASA Ames Research Center. Modeling of the planet’s thermosphere and escape processes referenced work from Harvard University, Stanford University, and the Royal Astronomical Society community, while implications for atmospheric chemistry engaged scientists at Institut d’Astrophysique de Paris, Max Planck Institute for Extraterrestrial Physics, and University of Leiden.

Cultural Impact and Research Significance

The system’s role as one of the first transiting exoplanet benchmarks influenced public outreach by Smithsonian Institution, American Museum of Natural History, and media coverage on BBC, The New York Times, and Nature (journal). It became a case study in curricula at University of California, Imperial College London, and Tokyo University, and has been cited in instrumentation proposals to National Science Foundation, European Research Council, and NASA programs. The star-and-planet pair continues to inform mission planning for observatories such as James Webb Space Telescope, PLATO, and CHEOPS and remains integral to comparative studies by consortia including the Exoplanet Exploration Program and the International Astronomical Union.

Category:Stars Category:Exoplanet host stars