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{{redirect|Dimidium|the value|half|C. dimidium|Caladenia dimidia}}
{{more footnotes|date=October 2012}}
{{Short description|The first exoplanet to be discovered around a main-sequence star}}
{{Planetbox begin

| name = ] b
{{Infobox planet
}}
| name = Dimidium/51 Pegasi b
{{Planetbox image
| symbol =
| image = 51 Pegasi b by Celestia.jpg
| image = Artist impression of the exoplanet 51 Pegasi b.jpg
| caption = 51 Pegasi b as rendered in ].
| image_size =
}}
| image_alt =
{{Planetbox star
| caption = An artist's impression of 51 Pegasi b (center) and its star (right).
| background =
| bgcolour =
| label_width = <!-- DISCOVERY -->
| discoverer = ] and<br />]
| discovery_site = ], France
| discovered = {{start date and age|6 October 1995}}
| discovery_method = ] (])
| alt_names = Dimidium
| perihelion = {{convert|0.0520|AU|km|abbr=on}}
| semimajor = {{convert|0.0527|±|0.0030|AU|km|abbr=on}}
| mean_orbit_radius =
| eccentricity = 0.013 ± 0.012
| aphelion = {{convert|0.0534|AU|km|abbr=on}}
| period = 4.230785 ± 0.000036 ]<br />101.5388 ]
| synodic_period =
| avg_speed = 136 ]
| star = ] | star = ]
| mean_radius = {{val|1.9|0.3|ul=Jupiter radius}}<ref>{{Cite journal |last1=Martins |first1=J. H. C. |last2=Santos |first2=N. C. |last3=Figueira |first3=P. |last4=Faria |first4=J. P. |last5=Montalto |first5=M. |last6=Boisse |first6=I. |last7=Ehrenreich |first7=D. |last8=Lovis |first8=C. |last9=Mayor |first9=M. |last10=Melo |first10=C. |last11=Pepe |first11=F. |last12=Sousa |first12=S. G. |last13=Udry |first13=S. |last14=Cunha |first14=D. |date=2015-04-01 |title=Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b |url=https://ui.adsabs.harvard.edu/abs/2015A&A...576A.134M |journal=Astronomy and Astrophysics |volume=576 |pages=A134 |doi=10.1051/0004-6361/201425298 |issn=0004-6361|arxiv=1504.05962 |bibcode=2015A&A...576A.134M }}</ref>
| constell = ]
| mass = ≥0.472 ± 0.039 {{Jupiter mass|link=yes}}
| RA = {{RA|22|57|28.0}}
| single_temperature = 1284 ± 19 ]
| DEC = {{DEC|+20|46|08}}
| app_mag = 5.49
| dist_ly = 50.9 ± 0.3
| dist_pc = 15.61 ± 0.09
| class = G2.5IVa ''or'' G4-5Va
| mass = 1.06
| radius = 1.237 ± 0.047
| temperature = 5571 ± 102
| metallicity = 0.20 ± 0.07
| age = 6.1-8.1
}}
{{Planetbox orbit
| semimajor = 0.0527 ± 0.0030
| semimajor_gm = 7.89
| periastron = 0.0520
| periastron_gm = 7.79
| apastron = 0.0534
| apastron_gm = 7.99
| eccentricity = 0.013 ± 0.012
| period = 4.230785 ± 0.000036
| period_hour = 101.5388
| speed = 136
| arg_peri = 58
| t_peri = 2,450,001.51 ± 0.61
| semi-amp = 55.94 ± 0.69
}}
{{Planetbox character
| minimum_mass = 0.472 ± 0.039
| temperature = 1284 ± 19
| stellar_flux = 480
| rotation_period = Synchronous
}}
{{Planetbox discovery
| discovery_date = 6 October 1995
| discoverers = ] and<br>]
| discovery_site= {{flagicon|France}} ]
| discovery_method = ] (])
| discovery_status = Published
}}
{{Planetbox catalog
| names = ]
}}
{{Planetbox reference
| star = 51+Peg
| planet = b
}} }}
'''51 Pegasi b''', officially named '''Dimidium''' {{IPAc-en|d|I|'|m|I|d|i|@|m}}, is an ] approximately {{Convert|50|ly|pc|abbr=off|lk=on}} away in the ] of ]. It was the first ] to be discovered ]ing a ] star,<ref name=HTUW>{{cite AV media | title=How the Universe Works 3 | volume=Jupiter: Destroyer or Savior? | date=2014 | publisher=]}}</ref> the ] ], and marked a breakthrough in astronomical research. It is the ] for a class of planets called ]s.<ref name="Wenz">{{cite journal |last1=Wenz |first1=John |title=Lessons from scorching hot weirdo-planets |journal=Knowable Magazine |publisher= Annual Reviews |date=10 October 2019 |doi=10.1146/knowable-101019-2|doi-access=free |url=https://knowablemagazine.org/article/physical-world/2019/hot-jupiter-formation-theories |access-date=4 April 2022 |language=en}}</ref>
{{Planetbox end}}


In 2017, traces of water were discovered in the planet's ].<ref>{{Cite web|url=https://phys.org/news/2017-02-atmosphere-hot-jupiter-exoplanet-pegasi.html|title=Water detected in the atmosphere of hot Jupiter exoplanet 51 Pegasi b|date=February 1, 2017|website=phys.org}}</ref> In 2019, the ] was awarded in part for the discovery of 51 Pegasi b.<ref name="nobel"/>
'''51 Pegasi b''' (abbreviated '''51 Peg b'''), sometimes though unofficially named '''Bellerophon''', is an ] approximately 50 ]s away in the ] of ]. 51 Pegasi b was the first planet to be discovered orbiting a ] star, the ] ] (the first exoplanets at all were discovered in 1992 by ] around pulsar ]), and marked a breakthrough in astronomical research. It is the prototype for a class of planets called ]s.


== Name == ==Name==
] is the ] of the host star. The planet was originally designated 51 Pegasi b by ] and ], who discovered the planet in 1995. The following year it was unofficially dubbed "Bellerophon" {{IPAc-en|b|E|'|l|Er|@|f|Q|n}} by astronomer ], who followed the ] of naming planets after ] and ]s (] is a figure from Greek mythology who rode the winged horse ]).<ref>{{Cite web |title=01.17.96 - Discovery of two new planets -- the second and third within the last three months -- proves they aren't rare in our galaxy |url=https://newsarchive.berkeley.edu/news/media/releases/96legacy/releases.96/14301.html |access-date=2024-12-31 |website=newsarchive.berkeley.edu}}</ref>
The name ''51 Pegasi b'' is the official astronomical name of the planet. As with all ]s, the 'b' is used to indicate that this planet was the first discovered orbiting its parent star. Further undiscovered planets of 51 Pegasi would be designated c, d, e, f, and so on. All extrasolar planets have ] letters to differentiate from companion stars in the system (which are designated with an ] letter).


In July 2014, the ] launched ], a process for giving proper names to certain ]s and their host stars.<ref>. IAU.org. 9 July 2014</ref> The process involved public nomination and voting for the new names.<ref>{{Cite web |url=http://nameexoworlds.iau.org/process |title=NameExoWorlds The Process |access-date=2015-09-05 |archive-date=2015-08-15 |archive-url=https://web.archive.org/web/20150815025117/http://www.nameexoworlds.iau.org/process }}</ref> In December 2015, the ] announced the winning name for this ] was Dimidium.<ref>, International Astronomical Union, 15 December 2015.</ref> The name was submitted by the {{lang-for|de|]|Astronomical Society of Lucerne}}, ]. 'Dimidium' is ] for 'half', referring to the planet's ] of approximately half the mass of ].<ref>{{Cite web |url=http://nameexoworlds.iau.org/names |title=NameExoWorlds The Approved Names |access-date=2015-12-21 |archive-date=2018-02-01 |archive-url=https://web.archive.org/web/20180201043609/http://nameexoworlds.iau.org/names }}</ref>
Though not officially recognized, 51 Pegasi b is sometimes referred to as "Bellerophon".<ref>http://jumk.de/astronomie/exoplanets/51-pegasi.shtml</ref> This name comes from the ] hero ], who tamed ] (the Winged Horse). This has direct relations with the constellation in which lies the planet (]). This name is usually used as the informal name to show the similarities to the planets of the ]. The same argument and association with the constellation of host star was used to suggest informal names for other extrasolar planets.<ref>{{cite arxiv | title=Naming the extrasolar planets | date=October 21, 2009 | eprint=0910.3989 | author1=Lyra | class=astro-ph.EP}}</ref>


== Discovery == == Discovery ==
] in ]]]
The ]'s discovery was announced on October 6, 1995, by ] and ] of the ] in the journal '']''.<ref>{{cite journal | author=Mayor, Michael | author2=Queloz, Didier | title=A Jupiter-mass companion to a solar-type star | journal=] | volume=378 |issue=6555 |date=1995 | pages=355–359| doi=10.1038/378355a0| bibcode=1995Natur.378..355M| s2cid=4339201 }}</ref> They used the ] with the ] on the ] telescope in France and made world headlines with their announcement. For this discovery, they were awarded the 2019 ].<ref name="nobel">{{cite web|url=https://www.nobelprize.org/prizes/physics/2019/summary/|title=The Nobel Prize in Physics 2019|publisher= ]|access-date= 8 October 2019}}</ref>


The planet was discovered using a sensitive ] that could detect the slight and regular ] changes in the star's ]s of around 70 metres per second. These changes are caused by the planet's ]al effects from just 7 million kilometres' distance from the star.
The ]'s discovery was announced on October 6, 1995, by ] and ] in '']'',<ref>{{cite journal | url= | author=Mayor, Michael; Queloz, Didier | title=A Jupiter-mass companion to a solar-type star | journal=] | volume=378 |issue=6555 |year=1995 | pages=355–359| doi=10.1038/378355a0| format= | bibcode=1995Natur.378..355M}}</ref> using the ] at the ] with the ].


Within a week of the announcement, the planet was confirmed by another team using the ] in ].<ref>{{cite journal|title=51 Pegasi|journal=]|volume=6251|pages=1|url=http://www.cbat.eps.harvard.edu/iauc/06200/06251.html#Item1|bibcode=1995IAUC.6251....1M|last1=Mayor|first1=M.|last2=Queloz|first2=D.|last3=Marcy|first3=G.|last4=Butler|first4=P.|last5=Noyes|first5=R.|last6=Korzennik|first6=S.|last7=Krockenberger|first7=M.|last8=Nisenson|first8=P.|last9=Brown|first9=T.|last10=Kennelly|first10=T.|last11=Rowland|first11=C.|last12=Horner|first12=S.|last13=Burki|first13=G.|last14=Burnet|first14=M.|last15=Kunzli|first15=M.|year=1995}}</ref>
]


==Physical characteristics==
On October 12, 1995, less than a week after the announcement of the discovery, confirmation came from ] of ] and ] of the ], using the ] at the ] near ] in ].
]
]
After its discovery, many teams confirmed the planet's existence and obtained more observations of its properties. It was discovered that the planet orbits the star in around four days. It is much closer to it than ] is to the Sun,<ref name=HTUW/> moves at an ] of {{convert|136|km/s|mph|abbr=unit}}, yet has a minimum mass about half that of Jupiter (about 150 times that of the ]). At the time, the presence of a huge world so close to its star was not compatible with theories of ] and was considered an anomaly. However, since then, numerous other "hot Jupiters" have been discovered<ref name=HTUW/> (such as ] and ]), and astronomers are revising their theories of planet formation to account for them by studying ].<ref name="Wenz"/>


Assuming the planet is perfectly grey with no greenhouse or tidal effects, and a ] of 0.1, the temperature would be {{convert|1265|K|C F|abbr=unit}}. This is between the predicted temperatures of ] and ] ({{convert|1180|K|C F}}–{{convert|1392|K|C F}}), before they were measured.<ref name=Renard>{{cite book |doi=10.1117/12.790494 |journal=Proceedings of SPIE |date=2008 |last1=Renard |first1=Stéphanie |last2=Absil |first2=Olivier |last3=Berger |first3=Jean-Philippe |last4=Bonfils |first4=Xavier |last5=Forveille |first5=Thierry |last6=Malbet |first6=Fabien |chapter=Prospects for near-infrared characterisation of hot Jupiters with the VLTI Spectro-Imager (VSI) | title=Optical and Infrared Interferometry |volume=7013 |pages=70132Z–70132Z–10 |arxiv=0807.3014 |chapter-url=http://orbi.ulg.ac.be/bitstream/2268/30158/1/SPIE_poster.pdf |bibcode=2008SPIE.7013E..2ZR |s2cid=119268109 }}</ref>
The planet was discovered using a sensitive ] that could detect the slight and regular velocity changes in the star's spectral lines of around 70 metres per second. These changes are caused by the planet's gravitational effects from just 7 million kilometres' distance from the star.


In the report of the discovery, it was initially speculated that 51 Pegasi b was the stripped core of a brown dwarf of a decomposed star and was therefore composed of heavy elements, but it is now believed to be a ]. It is sufficiently massive that its thick atmosphere is not blown away by the star's ].
This was the first discovery of an exoplanet orbiting a Sun-like star. It marked a turning point and forced astronomers to accept that giant planets could exist in short-period orbits. Once astronomers realized that it was worth looking for giant planets with the currently available technology, much more telescope time was devoted to radial velocity planet searches, and hence many more exoplanets in the ]'s neighborhood have been discovered.

== Physical characteristics ==

After its discovery, many teams confirmed the planet's existence and obtained more observations of its properties. It was discovered that the planet orbits the star in around 4 Earth days, is much closer to it than ] is to our Sun, moves at an ] of 136&nbsp;km/s, yet has a minimum mass about half that of ] (about 150 times that of the ]). At the time, the presence of a huge world so close to its star was not compatible with theories of ] and was considered an anomaly. However, since then, numerous other 'hot Jupiters' have been discovered (see ] and ], for example), and astronomers are revising their theories of planet formation to account for them by studying ].

] in ].]]

Assuming the planet is perfectly grey with no greenhouse or tidal effects, and a Bond albedo of 0.1, the temperature would be 1265 ] (approximately 1000 °C / 1800 °F). This is between the predicted temperatures of ] and ] (1180–1392 K), before they were measured.<ref name=Renard>{{cite arXiv | title=Proceedings of SPIE|year=2008| eprint=0807.3014 |class=astro-ph | last1=Renard | first1=S. | last2=Absil | first2=O. | last3=Berger | first3=J. -P. | last4=Bonfils | first4=X. | last5=Forveille | first5=T. | last6=Malbet | first6=F. | doi=10.1117/12.790494 | chapter=Prospects for near-infrared characterisation of hot Jupiters with the VLTI Spectro-Imager (VSI) | volume=7013 | pages=70132Z}}</ref>

In the discover paper it was initially speculated that 51 Pegasi b was the stripped core of a brown dwarf that formed in situ and was therefore composed of heavy elements, but it is now believed to be a ]. It is sufficiently massive that its thick atmosphere is not blown away by the star's ].


51 Pegasi b probably has a greater ] than that of Jupiter despite its lower mass. This is because its superheated atmosphere must be puffed up into a thick but tenuous layer surrounding it. Beneath this, the gases that make up the planet would be so hot that the planet would glow red. Clouds of ] may exist in the atmosphere. 51 Pegasi b probably has a greater ] than that of Jupiter despite its lower mass. This is because its superheated atmosphere must be puffed up into a thick but tenuous layer surrounding it. Beneath this, the gases that make up the planet would be so hot that the planet would glow red. Clouds of ] may exist in the atmosphere.
Line 92: Line 60:
The planet is ] to its star, always presenting the same face to it. The planet is ] to its star, always presenting the same face to it.


The planet (with ]) is deemed a candidate for direct imaging by ].<ref name=lucas>{{cite journal | title=Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo | year=2007 | last1=Lucas | first1=P. W. | last2=Hough | first2=J. H. | last3=Bailey | first3=J. A. | last4=Tamura | first4=M. | last5=Hirst | first5=E. | last6=Harrison | first6=D. | doi=10.1111/j.1365-2966.2008.14182.x | journal=] | volume=393 | pages=229 | arxiv=0807.2568 |bibcode = 2009MNRAS.393..229L }}</ref> It is also a candidate for "near-infrared characterisation.... with the VLTI Spectro-Imager".<ref name=Renard /> The planet (with ]) was deemed a candidate for aperture ] by ].<ref name=lucas>{{cite journal | title=Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo | date=2007 | last1=Lucas | first1=P. W. | last2=Hough | first2=J. H. | last3=Bailey | first3=J. A. | last4=Tamura | first4=M. | last5=Hirst | first5=E. | last6=Harrison | first6=D. | doi=10.1111/j.1365-2966.2008.14182.x | journal=] | volume=393 | issue=1 | pages=229–244 | doi-access=free | arxiv=0807.2568 |bibcode = 2009MNRAS.393..229L }}</ref> It is also a candidate for "near-infrared characterisation.... with the ] Spectro-Imager".<ref name=Renard />


==Claims of direct detection of visible light==
The earlier, rocky-planet model was utilized as a setting by ] in the story ''Exchange Rate''.
The first ever direct detection of the ] reflected from an exoplanet has been made by an international team of astronomers on 51 Pegasi b. The astronomers studied light from 51 Pegasi b using the High Accuracy Radial velocity Planet Searcher (]) instrument at the European Southern Observatory's ] in Chile.<ref></ref> This detection allowed the inference of a true mass of 0.46 Jupiter masses.<ref>{{cite journal |doi=10.1051/0004-6361/201425298|title=Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b|journal=Astronomy & Astrophysics|volume=576|pages=A134|date=2015|last1=Martins|first1=J. H. C.|last2=Santos|first2=N. C.|last3=Figueira|first3=P.|last4=Faria|first4=J. P.|last5=Montalto|first5=M.|last6=Boisse|first6=I.|last7=Ehrenreich|first7=D.|last8=Lovis|first8=C.|last9=Mayor|first9=M.|last10=Melo|first10=C.|last11=Pepe|first11=F.|last12=Sousa|first12=S. G.|last13=Udry|first13=S.|last14=Cunha|first14=D.|bibcode=2015A&A...576A.134M|arxiv = 1504.05962 |s2cid=119224213 }}</ref> The optical detection could not be replicated in 2020, implying the planet has an ] below 0.15.<ref>{{cite journal |doi=10.1051/0004-6361/202039271 |title=The GAPS Programme at TNG. XXIX. No detection of reflected light from 51 Peg b using optical high-resolution spectroscopy |journal=Astronomy & Astrophysics|volume=646|pages=A159|date=2020|last1=Scandariato|first1=G.|last2=Borsa|first2=F.|last3=Sicilia|first3=D.|last4=Malavolta|first4=L.|display-authors=et al.|bibcode=2021A&A...646A.159S|arxiv = 2012.10435 }}</ref> Measurements in 2021 have marginally detected a polarized reflected light signal, which, while they cannot place limits on the albedo without assumptions made about the scattering mechanisms, could suggest a high albedo.<ref>{{citation|arxiv=2101.07411|year=2021|title=Polarization of hot Jupiter systems: A likely detection of stellar activity and a possible detection of planetary polarization|doi=10.1093/mnras/stab172 |last1=Bailey |first1=Jeremy |last2=Bott |first2=Kimberly |last3=Cotton |first3=Daniel V. |last4=Kedziora-Chudczer |first4=Lucyna |last5=Zhao |first5=Jinglin |last6=Evensberget |first6=Dag |last7=Marshall |first7=Jonathan P. |last8=Wright |first8=Duncan |last9=Lucas |first9=P. W. |journal=Monthly Notices of the Royal Astronomical Society |volume=502 |issue=2 |pages=2331–2345 |doi-access=free }}</ref>

== See also ==


==See also==
* ] * ]
* ] * ]
* ] * ]
* ]


== References == ==References==
{{Reflist}} {{reflist|refs=}}
*{{cite journal | url=http://www.iop.org/EJ/article/0004-637X/646/1/505/64046.html | author=Butler ''et al.'' | title=Catalog of Nearby Exoplanets | journal=The ] | volume=646 | issue=1 | pages=505–522 | year=2006 | doi=10.1086/504701 | last2=Wright | first2=J. T. | last3=Marcy | first3=G. W. | last4=Fischer | first4=D. A. | last5=Vogt | first5=S. S. | last6=Tinney | first6=C. G. | last7=Jones | first7=H. R. A. | last8=Carter | first8=B. D. | last9=Johnson | first9=J. A. | last10 = McCarthy | first10=C. | last11=Penny | first11=A.J. | bibcode=2006ApJ...646..505B|arxiv = astro-ph/0607493 }} ()


==Further reading==
== External links ==
*{{cite journal | url=http://www.iop.org/EJ/article/0004-637X/646/1/505/64046.html | author=Butler | title=Catalog of Nearby Exoplanets | journal=The Astrophysical Journal | volume=646 | issue=1 | pages=505–522 | date=2006 | doi=10.1086/504701 | last2=Wright | first2=J. T. | last3=Marcy | first3=G. W. | last4=Fischer | first4=D. A. | last5=Vogt | first5=S. S. | last6=Tinney | first6=C. G. | last7=Jones | first7=H. R. A. | last8=Carter | first8=B. D. | last9=Johnson | first9=J. A. | last10=McCarthy | first10=C. | last11=Penny | first11=A.J. | bibcode=2006ApJ...646..505B | arxiv=astro-ph/0607493 | s2cid=119067572 | display-authors=etal | access-date=2009-03-11 | archive-date=2019-12-07 | archive-url=https://web.archive.org/web/20191207150418/http://www.iop.org/EJ/article/0004-637X/646/1/505/64046.html }} ()

==External links==
{{commons category}}
* {{cite encyclopedia |url=https://exoplanet.eu/catalog/51_peg_b--12/ |title=Notes for Planet 51 Peg b |author=Jean Schneider |date=2011 |encyclopedia=] |access-date=3 October 2011}}
* {{cite web |url=http://www.solstation.com/stars2/51pegasi.htm |title=51 Pegasi |access-date=2008-07-03 |work=SolStation | archive-url= https://web.archive.org/web/20080725012818/http://www.solstation.com/stars2/51pegasi.htm| archive-date= 25 July 2008 | url-status= live}}
* {{cite web | url=http://media4.obspm.fr/exoplanets/base/etoile.php?nom=51+Peg | title=51 Peg | work=Exoplanets | access-date=2008-11-01 | archive-url=https://web.archive.org/web/20081204060944/http://media4.obspm.fr/exoplanets/base/etoile.php?nom=51+Peg | archive-date=2008-12-04 }}
* {{cite web |url=http://obswww.unige.ch/~udry/planet/51peg.html |title=The First Extrasolar Planet around a Solar-type Star |access-date=2008-07-03 |work=] | archive-url= https://web.archive.org/web/20080609085714/http://obswww.unige.ch/~udry/planet/51peg.html| archive-date= 9 June 2008 | url-status= live}}
* {{cite web |url=http://exoplanets.org/esp/51peg/51peg.shtml |title=The Planet Around 51 Peg |access-date=2008-07-03 |work=] |archive-url=https://web.archive.org/web/20080727005731/http://exoplanets.org/esp/51peg/51peg.shtml |archive-date=2008-07-27 }}


* {{cite web |url=http://exoplanet.eu/planet.php?p1=51+Peg&p2=b |title=Notes for Planet 51 Peg b |author=Jean Schneider |year=2011 |work= |publisher=] |accessdate=3 October 2011}}
* {{cite web |url=http://www.solstation.com/stars2/51pegasi.htm |title=51 Pegasi |accessdate=2008-07-03 |work=SolStation |publisher= |date= | archiveurl= http://web.archive.org/web/20080725012818/http://www.solstation.com/stars2/51pegasi.htm| archivedate= 25 July 2008 <!--DASHBot-->| deadurl= no}}
* {{cite web | url=http://media4.obspm.fr/exoplanets/base/etoile.php?nom=51+Peg | title=51 Peg | work=Exoplanets}}
* {{cite web |url=http://www.extrasolar.net/planettour.asp?StarCatId=&PlanetId=1 |title="Bellerophon" - 51 Pegasi b |accessdate=2008-07-03 |work=Extrasolar Visions |publisher= |date= }}
* {{cite web |url=http://obswww.unige.ch/~udry/planet/51peg.html |title=The First Extrasolar Planet around a Solar-type Star |accessdate=2008-07-03 |work=] |publisher= |date= | archiveurl= http://web.archive.org/web/20080609085714/http://obswww.unige.ch/~udry/planet/51peg.html| archivedate= 9 June 2008 <!--DASHBot-->| deadurl= no}}
* {{cite web |url=http://exoplanets.org/esp/51peg/51peg.shtml |title=The Planet Around 51 Peg |accessdate=2008-07-03 |work=] |publisher= |date= | archiveurl= http://web.archive.org/web/20080727005731/http://exoplanets.org/esp/51peg/51peg.shtml| archivedate= 27 July 2008 <!--DASHBot-->| deadurl= no}}
{{Sky|22|57|28.0|+|20|46|08|50.1}} {{Sky|22|57|28.0|+|20|46|08|50.1}}
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Latest revision as of 11:40, 31 December 2024

"Dimidium" redirects here. For the value, see half. For C. dimidium, see Caladenia dimidia. The first exoplanet to be discovered around a main-sequence star
Dimidium/51 Pegasi b
An artist's impression of 51 Pegasi b (center) and its star (right).
Discovery
Discovered byMichel Mayor and
Didier Queloz
Discovery siteOHP, France
Discovery date6 October 1995; 29 years ago (6 October 1995)
Detection methodRadial velocity (ELODIE)
Designations
Alternative namesDimidium
Orbital characteristics
Aphelion0.0534 AU (7,990,000 km)
Perihelion0.0520 AU (7,780,000 km)
Semi-major axis0.0527 ± 0.0030 AU (7,880,000 ± 450,000 km)
Eccentricity0.013 ± 0.012
Orbital period (sidereal)4.230785 ± 0.000036 d
101.5388 h
Average orbital speed136 km/s
Star51 Pegasi
Physical characteristics
Mean radius1.9±0.3 RJ
Mass≥0.472 ± 0.039 MJ
Temperature1284 ± 19 K

51 Pegasi b, officially named Dimidium /dɪˈmɪdiəm/, is an extrasolar planet approximately 50 light-years (15 parsecs) away in the constellation of Pegasus. It was the first exoplanet to be discovered orbiting a main-sequence star, the Sun-like 51 Pegasi, and marked a breakthrough in astronomical research. It is the prototype for a class of planets called hot Jupiters.

In 2017, traces of water were discovered in the planet's atmosphere. In 2019, the Nobel Prize in Physics was awarded in part for the discovery of 51 Pegasi b.

Name

51 Pegasi is the Flamsteed designation of the host star. The planet was originally designated 51 Pegasi b by Michel Mayor and Didier Queloz, who discovered the planet in 1995. The following year it was unofficially dubbed "Bellerophon" /bɛˈlɛrəfɒn/ by astronomer Geoffrey Marcy, who followed the convention of naming planets after Greek and Roman mythological figures (Bellerophon is a figure from Greek mythology who rode the winged horse Pegasus).

In July 2014, the International Astronomical Union launched NameExoWorlds, a process for giving proper names to certain exoplanets and their host stars. The process involved public nomination and voting for the new names. In December 2015, the IAU announced the winning name for this planet was Dimidium. The name was submitted by the Astronomische Gesellschaft Luzern (German for 'Astronomical Society of Lucerne'), Switzerland. 'Dimidium' is Latin for 'half', referring to the planet's mass of approximately half the mass of Jupiter.

Discovery

The location of 51 Pegasi in Pegasus

The exoplanet's discovery was announced on October 6, 1995, by Michel Mayor and Didier Queloz of the University of Geneva in the journal Nature. They used the radial velocity method with the ELODIE spectrograph on the Observatoire de Haute-Provence telescope in France and made world headlines with their announcement. For this discovery, they were awarded the 2019 Nobel Prize in Physics.

The planet was discovered using a sensitive spectroscope that could detect the slight and regular velocity changes in the star's spectral lines of around 70 metres per second. These changes are caused by the planet's gravitational effects from just 7 million kilometres' distance from the star.

Within a week of the announcement, the planet was confirmed by another team using the Lick Observatory in California.

Physical characteristics

Profile of planet 51 Pegasi b by NASA
Promotional "Exoplanet Travel Bureau" poster from NASA

After its discovery, many teams confirmed the planet's existence and obtained more observations of its properties. It was discovered that the planet orbits the star in around four days. It is much closer to it than Mercury is to the Sun, moves at an orbital speed of 136 km/s (300,000 mph), yet has a minimum mass about half that of Jupiter (about 150 times that of the Earth). At the time, the presence of a huge world so close to its star was not compatible with theories of planet formation and was considered an anomaly. However, since then, numerous other "hot Jupiters" have been discovered (such as 55 Cancri and τ Boötis), and astronomers are revising their theories of planet formation to account for them by studying orbital migration.

Assuming the planet is perfectly grey with no greenhouse or tidal effects, and a Bond albedo of 0.1, the temperature would be 1,265 K (992 °C; 1,817 °F). This is between the predicted temperatures of HD 189733 b and HD 209458 b (1,180 K (910 °C; 1,660 °F)–1,392 K (1,119 °C; 2,046 °F)), before they were measured.

In the report of the discovery, it was initially speculated that 51 Pegasi b was the stripped core of a brown dwarf of a decomposed star and was therefore composed of heavy elements, but it is now believed to be a gas giant. It is sufficiently massive that its thick atmosphere is not blown away by the star's solar wind.

51 Pegasi b probably has a greater radius than that of Jupiter despite its lower mass. This is because its superheated atmosphere must be puffed up into a thick but tenuous layer surrounding it. Beneath this, the gases that make up the planet would be so hot that the planet would glow red. Clouds of silicates may exist in the atmosphere.

The planet is tidally locked to its star, always presenting the same face to it.

The planet (with Upsilon Andromedae b) was deemed a candidate for aperture polarimetry by Planetpol. It is also a candidate for "near-infrared characterisation.... with the VLTI Spectro-Imager".

Claims of direct detection of visible light

The first ever direct detection of the visible light spectrum reflected from an exoplanet has been made by an international team of astronomers on 51 Pegasi b. The astronomers studied light from 51 Pegasi b using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the European Southern Observatory's La Silla Observatory in Chile. This detection allowed the inference of a true mass of 0.46 Jupiter masses. The optical detection could not be replicated in 2020, implying the planet has an albedo below 0.15. Measurements in 2021 have marginally detected a polarized reflected light signal, which, while they cannot place limits on the albedo without assumptions made about the scattering mechanisms, could suggest a high albedo.

See also

References

  1. Martins, J. H. C.; Santos, N. C.; Figueira, P.; Faria, J. P.; Montalto, M.; Boisse, I.; Ehrenreich, D.; Lovis, C.; Mayor, M.; Melo, C.; Pepe, F.; Sousa, S. G.; Udry, S.; Cunha, D. (2015-04-01). "Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b". Astronomy and Astrophysics. 576: A134. arXiv:1504.05962. Bibcode:2015A&A...576A.134M. doi:10.1051/0004-6361/201425298. ISSN 0004-6361.
  2. ^ How the Universe Works 3. Vol. Jupiter: Destroyer or Savior?. Discovery Channel. 2014.
  3. ^ Wenz, John (10 October 2019). "Lessons from scorching hot weirdo-planets". Knowable Magazine. Annual Reviews. doi:10.1146/knowable-101019-2. Retrieved 4 April 2022.
  4. "Water detected in the atmosphere of hot Jupiter exoplanet 51 Pegasi b". phys.org. February 1, 2017.
  5. ^ "The Nobel Prize in Physics 2019". Nobel Media AB. Retrieved 8 October 2019.
  6. "01.17.96 - Discovery of two new planets -- the second and third within the last three months -- proves they aren't rare in our galaxy". newsarchive.berkeley.edu. Retrieved 2024-12-31.
  7. NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  8. "NameExoWorlds The Process". Archived from the original on 2015-08-15. Retrieved 2015-09-05.
  9. Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  10. "NameExoWorlds The Approved Names". Archived from the original on 2018-02-01. Retrieved 2015-12-21.
  11. Mayor, Michael; Queloz, Didier (1995). "A Jupiter-mass companion to a solar-type star". Nature. 378 (6555): 355–359. Bibcode:1995Natur.378..355M. doi:10.1038/378355a0. S2CID 4339201.
  12. Mayor, M.; Queloz, D.; Marcy, G.; Butler, P.; Noyes, R.; Korzennik, S.; Krockenberger, M.; Nisenson, P.; Brown, T.; Kennelly, T.; Rowland, C.; Horner, S.; Burki, G.; Burnet, M.; Kunzli, M. (1995). "51 Pegasi". IAU Circular. 6251: 1. Bibcode:1995IAUC.6251....1M.
  13. ^ Renard, Stéphanie; Absil, Olivier; Berger, Jean-Philippe; Bonfils, Xavier; Forveille, Thierry; Malbet, Fabien (2008). "Prospects for near-infrared characterisation of hot Jupiters with the VLTI Spectro-Imager (VSI)" (PDF). Optical and Infrared Interferometry. Vol. 7013. pp. 70132Z–70132Z–10. arXiv:0807.3014. Bibcode:2008SPIE.7013E..2ZR. doi:10.1117/12.790494. S2CID 119268109. {{cite book}}: |journal= ignored (help)
  14. Lucas, P. W.; Hough, J. H.; Bailey, J. A.; Tamura, M.; Hirst, E.; Harrison, D. (2007). "Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo". Monthly Notices of the Royal Astronomical Society. 393 (1): 229–244. arXiv:0807.2568. Bibcode:2009MNRAS.393..229L. doi:10.1111/j.1365-2966.2008.14182.x.
  15. physicsworld.com 2015-04-22 First visible light detected directly from an exoplanet
  16. Martins, J. H. C.; Santos, N. C.; Figueira, P.; Faria, J. P.; Montalto, M.; Boisse, I.; Ehrenreich, D.; Lovis, C.; Mayor, M.; Melo, C.; Pepe, F.; Sousa, S. G.; Udry, S.; Cunha, D. (2015). "Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b". Astronomy & Astrophysics. 576: A134. arXiv:1504.05962. Bibcode:2015A&A...576A.134M. doi:10.1051/0004-6361/201425298. S2CID 119224213.
  17. Scandariato, G.; Borsa, F.; Sicilia, D.; Malavolta, L.; et al. (2020). "The GAPS Programme at TNG. XXIX. No detection of reflected light from 51 Peg b using optical high-resolution spectroscopy". Astronomy & Astrophysics. 646: A159. arXiv:2012.10435. Bibcode:2021A&A...646A.159S. doi:10.1051/0004-6361/202039271.
  18. Bailey, Jeremy; Bott, Kimberly; Cotton, Daniel V.; Kedziora-Chudczer, Lucyna; Zhao, Jinglin; Evensberget, Dag; Marshall, Jonathan P.; Wright, Duncan; Lucas, P. W. (2021), "Polarization of hot Jupiter systems: A likely detection of stellar activity and a possible detection of planetary polarization", Monthly Notices of the Royal Astronomical Society, 502 (2): 2331–2345, arXiv:2101.07411, doi:10.1093/mnras/stab172

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