View source for HD 209458

{{Short description|F-type or G-type star in the constellation Pegasus}}
{{Starbox begin
 | name = HD 209458
}}
{{Starbox image
 | image = {{Location map|100x100|AlternativeMap=Pegasus constellation map.svg|width=250|lat=49.0|long=66.6|mark=Cercle rouge 100%.svg | marksize=14|float=center}}
 | caption = Location of HD 209458 in the constellation Pegasus
}}
{{Starbox observe
 | epoch       = J2000
 | constell    = [[Pegasus (constellation)|Pegasus]]<ref name=Anderson_Francis_2012/>
 | ra          = {{RA|22|03|10.77275}}<ref name="GaiaDR3"/>
 | dec         = {{DEC|+18|53|03.5494}}<ref name="GaiaDR3"/>
 | appmag_v    = 7.65<ref name=Anderson_Francis_2012/>
}}
{{Starbox character
 | type        = main sequence<ref name="GaiaDR3"/>
 | class       = F9&nbsp;V<ref name=Gray_et_al_2001/> or G0&nbsp;V<ref name="Charbonneau2000"/>
 | appmag_1_passband = B
 | appmag_1    = 8.244<ref name=Anderson_Francis_2012/>
 | appmag_2_passband = K
 | appmag_2    = 6.308{{±|0.026}}<ref name=Anderson_Francis_2012/>
 | b-v         = +0.574{{±|0.014}}<ref name="mazeh"/>
 | u-b         = 
 | variable    = [[Planetary transit variable|EP]]<ref name="gcvs"/>
}}
{{Starbox astrometry
 | radial_v    = {{val|−14.78|0.16}}<ref name="GaiaDR3"/>
 | prop_mo_ra  = {{val|29.766|(28)}}
 | prop_mo_dec = {{val|−17.976|(25)}}
 | pm_footnote = <ref name="GaiaDR3"/>
 | parallax    = 20.7694
 | p_error     = 0.0266
 | parallax_footnote = <ref name="GaiaDR3"/>
 | absmag_v    = 4.28{{±|0.10}}<ref name="mazeh"/>
}}
{{Starbox detail
 | mass        = {{val|1.148|0.022}}<ref name="Del Burgo2016"/>
 | radius      = {{val|1.203|0.061}}<ref name="Del Burgo2016"/>
 | luminosity  = {{val|1.77|0.14}}<ref name="Del Burgo2016"/>
 | temperature = {{val|fmt=commas|6071|20}}<ref name="Del Burgo2016"/>
 | gravity     = 4.38<ref name="Del Burgo2016"/>
 | metal_fe    = 0.00{{±|0.02}}<ref name="mazeh"/>
 | rotation    = {{val|10.65|0.75|ul=d}}<ref name="Bonomo2017"/>
 | rotational_velocity = {{val|4.49|0.50}}<ref name="Bonomo2017"/>
 | age_gyr     = {{val|3.5|1.4}}<ref name="Del Burgo2016"/>
}}
{{Starbox catalog
 | names = {{odlist | 2MASS=J22031077+1853036 | BD=+18°4917 | HIP=108859 | SAO=107623 | V=V376 Peg }}<ref name="SIMBAD"/>
}}
{{Starbox reference
 | Simbad = HD+209458
 | NSTED = HD209458 
 | EPE = HD+209458
}}
{{Starbox end}}

'''HD 209458''' is a [[star]] with an orbiting [[exoplanet]] in the [[constellation]] [[Pegasus (constellation)|Pegasus]]. It has an [[apparent visual magnitude]] of 7.65<ref name=Anderson_Francis_2012/> and an [[absolute magnitude]] of 4.28.<ref name="mazeh"/> Because it is located at a distance of {{convert|157|ly|pc|lk=on|abbr=off}} from the Sun as measured via [[stellar parallax|parallax]], it is not visible to the unaided eye. With good [[binoculars]] or a small [[telescope]] it should be easily detectable. The system is drifting closer with a heliocentric [[radial velocity]] of −14.8&nbsp;km/s.<ref name="GaiaDR3"/>

==Stellar properties==
[[File:V376PegLightCurve.png|thumb|left|A [[light curve]] of planet HD&nbsp;209458 b transiting the star, adapted from Brown ''et al.'' (2001)<ref name="BrownHST"/>]]
The [[stellar spectrum|spectrum]] of HD 209458 presents as a [[late type star|late]] [[F-type main-sequence star|F-]] or early [[G-type main-sequence star]] with a [[stellar classification]] of F9&nbsp;V<ref name=Gray_et_al_2001/> or G0&nbsp;V,<ref name="Charbonneau2000"/> respectively. It is roughly 3.5<ref name="Del Burgo2016"/>&nbsp;billion years old and is spinning with a [[projected rotational velocity]] of 4.2&nbsp;km/s.<ref name="Bonomo2017"/> The star displays a moderate amount of [[magnetic activity]] in its [[chromosphere]].<ref name=Ben-Jaffel_2007/> It has a 15% greater mass than the Sun and a 20% larger radius. The abundance of iron, a measure of the [[metallicity]] of the star, is solar.<ref name="Del Burgo2016"/> HD 209458 is radiating 1.8 times the luminosity of the Sun from its [[photosphere]] at an [[effective temperature]] of 6,071&nbsp;K.<ref name="Del Burgo2016"/>

Because the planet transits the star, the star is dimmed by about 2% every 3.5 days making it an [[variable star#Extrinsic variables|extrinsic variable]]. The [[variable star]] designation for HD&nbsp;209458 is '''V376&nbsp;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.<ref name="gcvs"/><ref name=vsx/>

==Planetary system==
[[Image:HD209458.jpg|thumb|left|HD 209458 (north is towards top right)]]
In 1999, two teams working independently (one team consisted of astronomers at the [[Geneva Observatory]], the [[Harvard-Smithsonian Center for Astrophysics|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]] [[Radial velocity method|search method]]. Soon after the discovery, separate teams led by [[David Charbonneau]] and [[Gregory W. Henry]] were able to detect a [[transit (astronomy)|transit]] of the planet across the surface of the star making it the first known transiting extrasolar planet.<ref name="Charbonneau2000"/><ref name="Keck"/> The planet received the designation [[HD 209458 b]].

The planet is now under even more public scrutiny with the announcement that its atmosphere contains water vapor.  Astronomers had made careful [[photometry (astronomy)|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 [[astronomical transit|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,<ref name="Barman2007"/> although previous research in 2007<ref name="Richardson2007"/> 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.<ref name="Santos2020"/> A later study in 2021 did not find any molecular absorption features in the planetary atmosphere at all.<ref name="Casasayas-Barris2021"/>

{{OrbitboxPlanet begin
 | table_ref = <ref name="Bonomo2017"/>
}}
{{OrbitboxPlanet
 | exoplanet    = [[HD 209458 b|b]]
 | mass         = {{val|0.682|0.014|0.015}}
 | period       = {{val|3.52474859|(38)}}
 | semimajor    = {{val|0.04707|0.00045|0.00047}}
 | eccentricity = <0.0081
 | inclination  = {{val|86.71|0.05}}
 | radius       = {{val|1.359|0.016|0.019}}
}}
{{Orbitbox end}}

==See also==
* [[List of exoplanets discovered before 2000]]

==References==
{{reflist|refs=

<ref name="SIMBAD">{{cite simbad | title=HD 209458 | access-date=2009-05-17 }}</ref>

<ref name="GaiaDR3">{{Cite Gaia DR3|1779546757669063552}}</ref>

<ref name=Anderson_Francis_2012>{{cite XHIP|108859}}</ref>

<ref name=vsx>{{cite web|url=http://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt|title=GCVS Variability Types and Distribution Statistics of Designated Variable Stars According to their Types of Variability|author=Samus, N. N.|date=2009|access-date=2010-08-02}}</ref>

<ref name=Gray_et_al_2001>{{cite journal
 | title=The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
 | display-authors=1 | last1=Gray | first1=R. O.
 | last2=Napier | first2=M. G. | last3=Winkler | first3=L. I.
 | journal=The Astronomical Journal
 | volume=121 | issue=4 | pages=2148 | year=2001
 | doi=10.1086/319956 | bibcode=2001AJ....121.2148G
 | s2cid=117076031 | doi-access= }}</ref>

<ref name="Barman2007">{{cite journal
 | title=Identification of Absorption Features in an Extrasolar Planet Atmosphere
 | last=Barman | first=T.
 | journal=The Astrophysical Journal Letters
 | volume=661 | issue=2 | date=2007 | pages=L191–L194
 | doi=10.1086/518736 | bibcode=2007ApJ...661L.191B
 | arxiv=0704.1114 | s2cid=13964464 }}</ref>

<ref name="Bonomo2017">{{cite journal
 | title=The GAPS Programme with HARPS-N at TNG. XIV. Investigating giant planet migration history via improved eccentricity and mass determination for 231 transiting planets
 | last1=Bonomo |first1=A. S. |last2=Desidera |first2=S.
 | last3=Benatti | first3=S. | last4=Borsa | first4=F.
 | last5=Crespi | first5=S. | last6=Damasso | first6=M.
 | last7=Lanza | first7=A. F. | last8=Sozzetti | first8=A.
 | last9=Lodato | first9=G. | last10=Marzari | first10=F.
 | last11=Boccato | first11=C. | last12=Claudi | first12=R. U.
 | last13=Cosentino | first13=R. | last14=Covino | first14=E.
 | last15=Gratton | first15=R. | last16=Maggio | first16=A.
 | last17=Micela | first17=G. | last18=Molinari | first18=E.
 | last19=Pagano | first19=I. | last20=Piotto | first20=G.
 | last21=Poretti | first21=E. | last22=Smareglia | first22=R.
 | last23=Affer | first23=L. | last24=Biazzo | first24=K.
 | last25=Bignamini | first25=A. | last26=Esposito | first26=M.
 | last27=Giacobbe | first27=P. | last28=Hébrard | first28=G.
 | last29=Malavolta | first29=L. | last30=Maldonado | first30=J.
 | last31=Mancini | first31=L. | last32=Martinez Fiorenzano | first32=A.
 | last33=Masiero | first33=S. | last34=Nascimbeni | first34=V.
 | last35=Pedani | first35=M. | last36=Rainer | first36=M.
 | last37=Scandariato | first37=G. | display-authors=1
 | journal=[[Astronomy & Astrophysics]]
 | volume=602 | issue= | pages=A107 | date=June 2017
 | doi=10.1051/0004-6361/201629882 |arxiv=1704.00373
 | bibcode=2017A&A...602A.107B}}</ref>

<ref name="BrownHST">{{cite journal
 | title=Hubble Space Telescope Time-Series Photometry of the Transiting Planet of HD 209458
 | display-authors=1 | last1=Brown | first1=Timothy M.
 | last2=Charbonneau | first2=David | last3=Gilliland | first3=Ronald L.
 | last4=Noyes | first4=Robert W. | last5=Burrows | first5=Adam
 | journal=The Astrophysical Journal
 | date=May 2001 | volume=552 | issue=2 | pages=699–709
 | doi=10.1086/320580 | arxiv=astro-ph/0101336
 | bibcode=2001ApJ...552..699B | s2cid=51819558 }}</ref>

<ref name="Casasayas-Barris2021">{{cite journal
 | title=The atmosphere of HD 209458b seen with ESPRESSO
 | last1=Casasayas-Barris | first1=N. | last2=Palle | first2=E.
 | last3=Stangret | first3=M. | last4=Bourrier | first4=V.
 | last5=Tabernero | first5=H. M. | last6=Yan | first6=F.
 | last7=Borsa | first7=F. | last8=Allart | first8=R.
 | last9=Zapatero Osorio | first9=M. R. | last10=Lovis | first10=C.
 | last11=Sousa | first11=S. G. | last12=Chen | first12=G.
 | last13=Oshagh | first13=M. | last14=Santos | first14=N. C.
 | last15=Pepe | first15=F. | last16=Rebolo | first16=R.
 | last17=Molaro | first17=P. | last18=Cristiani | first18=S.
 | last19=Adibekyan | first19=V. | last20=Alibert | first20=Y.
 | last21=Allende Prieto | first21=C. | last22=Bouchy | first22=F.
 | last23=Demangeon | first23=O. D. S. | last24=Di Marcantonio | first24=P.
 | last25=d'Odorico | first25=V. | last26=Ehrenreich | first26=D.
 | last27=Figueira | first27=P. | last28=Génova Santos | first28=R.
 | last29=González Hernández | first29=J. I. | last30=Lavie | first30=B.
 | display-authors=1 | journal=Astronomy & Astrophysics
 | volume=647 | pages=A26 | year=2021
 | s2cid=231573096 | doi=10.1051/0004-6361/202039539
 | arxiv=2101.04094 | bibcode=2021A&A...647A..26C }}</ref>

<ref name="Charbonneau2000">{{cite journal
 | title=Detection of Planetary Transits Across a Sun-like Star
 | last1=Charbonneau | first1=David | last2=Brown | first2=Timothy M.
 | last3=Latham | first3=David W. | last4=Mayor | first4=Michel
 | display-authors=1 | journal=The Astrophysical Journal Letters
 | volume=529 | issue=1 | pages=L45–L48 | year=2000
 | arxiv=astro-ph/9911436 | bibcode=2000ApJ...529L..45C
 | doi=10.1086/312457 | pmid=10615033 | s2cid=16152423 }}</ref>

<ref name="Del Burgo2016">{{cite journal
 | title=Accurate parameters for HD 209458 and its planet from HST spectrophotometry
 | last1=Del Burgo | first1=C. | last2=Allende Prieto | first2=C.
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=463 | issue=2 | pages=1400–1408 | year=2016
 | arxiv=1703.01449 | bibcode=2016MNRAS.463.1400D
 | doi=10.1093/mnras/stw2005 | doi-access=free | s2cid=119244708 }}</ref>

<ref name="gcvs">{{cite web
 | url=http://www.sai.msu.su/groups/cluster/gcvs/cgi-bin/search.cgi?search=V376+Peg
 | title=GCVS query result: V376 Peg 
 | work=[[General Catalog of Variable Stars]]
 | publisher=Sternberg Astronomical Institute, Moscow, Russia
 | access-date=2009-05-17 }}</ref>

<ref name="Keck">{{cite press release
 | title=Astronomers witness silhouette of planet crossing distant star
 | date=November 7, 1999 | publisher=W. M. Keck Observatory
 | location=Kamuela, Hawaii
 | url=http://www.keckobservatory.org/astronomers_witness_silhouette_of_planet_crossing_distant_star/
 | access-date=August 13, 2019 }}</ref>

<ref name="mazeh">{{cite journal 
 | title=The Spectroscopic Orbit of the Planetary Companion Transiting HD 209458
 | first1=Tsevi | last1=Mazeh | last2=Naef | first2=Dominique 
 | last3=Torres | first3=Guillermo | last4=Latham | first4=David W. 
 | last5=Mayor | first5=Michel | last6=Beuzit | first6=Jean-Luc 
 | last7=Brown | first7=Timothy M. | last8=Buchhave | first8=Lars 
 | last9=Burnet | first9=Michel | display-authors=1 
 | journal=The Astrophysical Journal
 | date=2000 | volume=532 | pages=L55–L58 | issue=1
 | doi=10.1086/312558 | s2cid=6139784 | arxiv=astro-ph/0001284 
 | pmid=10702131| bibcode=2000ApJ...532L..55M }}</ref>

<ref name="Richardson2007">{{cite journal
 | title=A spectrum of an extrasolar planet
 | display-authors=1 | first1=L. J. | last1=Richardson
 | last2=Deming | first2=Drake | last3=Horning | first3=Karen
 | last4=Seager | first4=Sara | last5=Harrington | first5=Joseph
 | journal=Nature | date=2007
 | volume=445 | pages=892–895 | issue=7130
 | doi=10.1038/nature05636 | pmid=17314975 | s2cid=4415500
 | bibcode=2007Natur.445..892R | arxiv = astro-ph/0702507 }}</ref>

<ref name="Santos2020">{{cite journal
 | title=Broadband transmission spectroscopy of HD 209458b with ESPRESSO: Evidence for Na, TiO, or both
 | last1=Santos | first1=N. C. | last2=Cristo | first2=E.
 | last3=Demangeon | first3=O. | last4=Oshagh | first4=M.
 | last5=Allart | first5=R. | last6=Barros | first6=S. C. C.
 | last7=Borsa | first7=F. | last8=Bourrier | first8=V.
 | last9=Casasayas-Barris | first9=N. | last10=Ehrenreich | first10=D.
 | last11=Faria | first11=J. P. | last12=Figueira | first12=P.
 | last13=Martins | first13=J. H. C. | last14=Micela | first14=G.
 | last15=Pallé | first15=E. | last16=Sozzetti | first16=A.
 | last17=Tabernero | first17=H. M. | last18=Zapatero Osorio | first18=M. R.
 | last19=Pepe | first19=F. | last20=Cristiani | first20=S.
 | last21=Rebolo | first21=R. | last22=Adibekyan | first22=V.
 | last23=Allende Prieto | first23=C.|last24=Alibert | first24=Y.
 | last25=Bouchy | first25=F. | last26=Cabral | first26=A.
 | last27=Dekker | first27=H. | last28=Di Marcantonio | first28=P.
 | last29=d'Odorico | first29=V. | last30=Dumusque | first30=X.
 | display-authors=1 | journal=Astronomy & Astrophysics
 | volume=644 | pages=A51 | year=2020
 | doi=10.1051/0004-6361/202039454 | arxiv=2011.03746
 | bibcode=2020A&A...644A..51S | s2cid=226281853 }}</ref>

<ref name=Ben-Jaffel_2007>{{cite journal
 | title=Exoplanet HD 209458b: Inflated Hydrogen Atmosphere but No Sign of Evaporation
 | last=Ben-Jaffel | first=Lotfi
 | journal=The Astrophysical Journal
 | volume=671 | issue=1 | pages=L61–L64 | date=December 2007
 | doi=10.1086/524706 | arxiv=0711.1432
 | bibcode=2007ApJ...671L..61B }}</ref>

}}

==External links==
* {{cite web
| url=http://media4.obspm.fr/exoplanets/base/etoile.php?nom=HD+209458
| title=HD 209458
| work=Exoplanets
| access-date=2009-05-17
| archive-url=https://web.archive.org/web/20091125091136/http://media4.obspm.fr/exoplanets/base/etoile.php?nom=HD+209458
| archive-date=2009-11-25
| url-status=dead
}}

{{Sky|22|03|10.8||18|53|04|154}}
{{Stars of Pegasus}}

{{DEFAULTSORT:HD 209458}}
<!-- Properties -->
[[Category:F-type main-sequence stars]]
[[Category:G-type main-sequence stars]]
[[Category:Planetary transit variables]]
[[Category:Planetary systems with one confirmed planet]]
<!-- Location/catalogues -->
[[Category:Pegasus (constellation)]]
[[Category:Durchmusterung objects|BD+18 4917]]
[[Category:Henry Draper Catalogue objects|209458]]
[[Category:Hipparcos objects|108859]]
[[Category:Objects with variable star designations|Pegasi, V376]]
[[Category:2MASS objects|J22031077+1853036]]
[[Category:Population I stars]]