HD 209458

HD 209458 is an 8th-magnitude star in the constellation Pegasus. It is a G0V star, and is thus very similar to the Sun. Because it is located at a distance of 157 light-years (48 parsecs), it is not visible to the unaided eye. With good binoculars or a small telescope it should be easily detectable.[2]

A light curve of planet HD 209458 b transiting the star, adapted from Brown et al. (2001)[6]
HD 209458
HD 209458 is located in 100x100
HD 209458
HD 209458 (100x100)

HD 209458 (circled) in the constellation Pegasus.
Observation data
Epoch J2000      Equinox J2000
Constellation Pegasus
Right ascension 22h 03m 10.77275s[1]
Declination +18° 53′ 03.5494″[1]
Apparent magnitude (V) 7.65[2]
Characteristics
Spectral type G0V[2]
Apparent magnitude (B) 8.24[2]
Apparent magnitude (K) 6.3[2]
B−V color index +0.574 ±0.014[3]
Variable type EP[4]
Astrometry
Radial velocity (Rv)−14.78±0.16[1] km/s
Proper motion (μ) RA: 29.766(28) mas/yr[1]
Dec.: −17.976(25) mas/yr[1]
Parallax (π)20.7694 ± 0.0266 mas[1]
Distance157.0 ± 0.2 ly
(48.15 ± 0.06 pc)
Absolute magnitude (MV)4.28 ±0.10[3]
Details
Mass1.148±0.022[5] M☉
Radius1.203±0.061[5] R☉
Luminosity1.77±0.14[5] L☉
Temperature6071±20[5] K
Metallicity [Fe/H]0.00 ± 0.02[3] dex
Rotation14.4 days
Age3.5±1.4[5] Gyr
Other designations
BD +18°4917, SAO 107623, V376 Peg, HIP 108859, 2MASS J22031077+1853036.
Database references
SIMBADdata
Exoplanet Archivedata
Extrasolar Planets
Encyclopaedia		data
HD 209458

In 1999, two teams working independently (one team consisted of astronomers at the Geneva Observatory, the Center for Astrophysics | Harvard & Smithsonian, 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.[7][8] The planet received the designation HD 209458 b.

Because the planet transits the star, the star is dimmed by about 2% every 3.5 days making it an extrinsic variable. The variable star designation for HD 209458 is V376 Pegasi. It is the prototype of the variable class "EP" in the General Catalogue of Variable Stars, defined as stars showing eclipses by their planets.[4][9]

Planetary system

HD 209458 b is an extrasolar planet that orbits the Sun-like star HD 209458 in the constellation Pegasus. Spectroscopic studies first revealed the presence of a planet around HD 209458 on November 5, 1999. The planet is now under even more public scrutiny with the announcement that its atmosphere contains water vapor. 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.

Travis Barman at Lowell Observatory in Flagstaff, Arizona analyzed the emission spectrum of this planet in 2007, and believes that its atmosphere contains water vapor,[10] although previous research in 2007[11] suggests that the atmosphere is composed mostly of silicate clouds. A spectrum taken in 2020 detected either sodium or titanium oxide in the planet's atmosphere.[12] A later study in 2021 did not find any molecular absorption features in the planetary atmosphere at all.[13]

The HD 209458 planetary system[14]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b 0.682+0.014
−0.015 MJ		 0.04707+0.00045
−0.00047		 3.52474859(38) <0.0081 86.71±0.05° 1.359+0.016
−0.019 RJ

References
  1. Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. "V* V376 Peg". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2009-05-17.
  3. Tsevi Mazeh, et al. (2000). "The Spectroscopic Orbit of the Planetary Companion Transiting HD 209458" (PDF). The Astrophysical Journal. 532 (1): L55–L58. arXiv:astro-ph/0001284. Bibcode:2000ApJ...532L..55M. doi:10.1086/312558. PMID 10702131. S2CID 6139784.
  4. "GCVS query result: V376 Peg". General Catalog of Variable Stars. Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2009-05-17.
  5. Del Burgo, C; Allende Prieto, C (2016). "Accurate parameters for HD 209458 and its planet from HST spectrophotometry". Monthly Notices of the Royal Astronomical Society. 463 (2): 1400–1408. arXiv:1703.01449. Bibcode:2016MNRAS.463.1400D. doi:10.1093/mnras/stw2005. S2CID 119244708.
  6. Brown, Timothy M.; Charbonneau, David; Gilliland, Ronald L.; Noyes, Robert W.; Burrows, Adam (May 2001). "Hubble Space Telescope Time-Series Photometry of the Transiting Planet of HD 209458". The Astrophysical Journal. 552 (2): 699–709. arXiv:astro-ph/0101336. Bibcode:2001ApJ...552..699B. doi:10.1086/320580. S2CID 51819558. Retrieved 14 February 2022.
  7. Charbonneau, David; et al. (2000). "Detection of Planetary Transits Across a Sun-like Star". The Astrophysical Journal Letters. 529 (1): L45–L48. arXiv:astro-ph/9911436. Bibcode:2000ApJ...529L..45C. doi:10.1086/312457. PMID 10615033. S2CID 16152423.
  8. "Astronomers witness silhouette of planet crossing distant star" (Press release). Kamuela, Hawaii: W. M. Keck Observatory. November 7, 1999. Retrieved August 13, 2019.
  9. Samus, N. N. (2009). "GCVS Variability Types and Distribution Statistics of Designated Variable Stars According to their Types of Variability". Retrieved 2010-08-02.
  10. Barman (2007). "Identification of Absorption Features in an Extrasolar Planet Atmosphere". The Astrophysical Journal Letters. 661 (2): L191–L194. arXiv:0704.1114. Bibcode:2007ApJ...661L.191B. doi:10.1086/518736. S2CID 13964464.
  11. L. J. Richardson, et al. (2007). "A spectrum of an extrasolar planet". Nature. 445 (7130): 892–895. arXiv:astro-ph/0702507. Bibcode:2007Natur.445..892R. doi:10.1038/nature05636. PMID 17314975. S2CID 4415500.
  12. Santos, N. C.; et al. (2020), "Broadband transmission spectroscopy of HD 209458b with ESPRESSO: Evidence for Na, TiO, or both", Astronomy & Astrophysics, 644: A51, arXiv:2011.03746, Bibcode:2020A&A...644A..51S, doi:10.1051/0004-6361/202039454, S2CID 226281853
  13. Casasayas-Barris, N.; et al. (2021), "The atmosphere of HD 209458b seen with ESPRESSO", Astronomy & Astrophysics, 647: A26, arXiv:2101.04094, doi:10.1051/0004-6361/202039539, S2CID 231573096
  14. Bonomo, A. S.; Desidera, S.; et al. (June 2017). "The GAPS Programme with HARPS-N at TNG. XIV. Investigating giant planet migration history via improved eccentricity and mass determination for 231 transiting planets". Astronomy & Astrophysics. 602: A107. arXiv:1704.00373. Bibcode:2017A&A...602A.107B. doi:10.1051/0004-6361/201629882.
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