KELT-9b#Host star

The host star, KELT-9, is 2 to 3 times larger and 2 to 3 times more massive than the sun. The surface temperature is {{convert|10170|K|C F|0|lk=on|abbr=on}}, unusually hot for a star with a transiting planet. Prior to the discovery of KELT-9b, only six A-type stars were known to have planets, of which the warmest, WASP-33, is significantly cooler at {{convert|7430|K|C F|0|abbr=on}}; no B-type stars were previously known to host planets. KELT-9, classified as B9.5-A0 could be the first B-type star known to have a planet. KELT-9b occupies a circular but strongly inclined orbit a mere 0.03462 AU from KELT-9 with an orbital period of less than 1.5 days.[https://exoplanets.nasa.gov/exoplanet-catalog/3508/kelt-9-b/ KELT-9 b] Exoplanet Exploration Program 2017

File:PIA21472 - Hottest Hot Jupiter Animation (Artist's Concept).gif

KELT-9b is a relatively large giant planet at about 2.8 times the mass of Jupiter; however given that its radius is nearly twice that of Jupiter, its density is less than half that of it. Like many hot Jupiters, KELT-9b is tidally locked with its host star.[https://www.jpl.nasa.gov/news/news.php?feature=6865 NASA JPL, Pasadena CA (5 June 2017) Astronomers Find Planet Hotter Than Most Stars]

The outer boundary of its atmosphere nearly reaches its Roche lobe, implying that the planet is experiencing rapid atmospheric escape driven by the extreme amount of radiation it receives from its host star. In 2020, atmospheric loss rate was measured to be equal to 18 - 68 Earth masses per billion years.{{cite journal|arxiv=2004.13733|last1=Wyttenbach|first1=A.|last2=Mollière|first2=P.|last3=Ehrenreich|first3=D.|last4=Cegla|first4=H. M.|last5=Bourrier|first5=V.|last6=Lovis|first6=C.|last7=Pino|first7=L.|last8=Allart|first8=R.|last9=Seidel|first9=J. V.|last10=Hoeijmakers|first10=H. J.|last11=Nielsen|first11=L. D.|last12=Lavie|first12=B.|last13=Pepe|first13=F.|last14=Bonfils|first14=X.|last15=Snellen|first15=I. A. G.|title=Mass loss rate and local thermodynamic state of KELT-9 b thermosphere from the hydrogen Balmer series|journal=Astronomy & Astrophysics|year=2020|volume=638|pages=A87|doi=10.1051/0004-6361/201937316|bibcode=2020A&A...638A..87W|s2cid=216641961}}

The planet's elemental abundances remain largely unknown as of 2022, but a low carbon-to-oxygen ratio is strongly suspected.{{citation|arxiv=2211.10297|year=2022|title=A strong H− opacity signal in the near-infrared emission spectrum of the ultra-hot Jupiter KELT-9b|last1=Jacobs |first1=Bob |last2=Désert |first2=Jean-Michel |last3=Pino |first3=Lorenzo |last4=Line |first4=Michael R. |last5=Bean |first5=Jacob L. |last6=Khorshid |first6=Niloofar |last7=Schlawin |first7=Everett |last8=Arcangeli |first8=Jacob |last9=Barat |first9=Saugata |last10=Jens Hoeijmakers |first10=H. |last11=Komacek |first11=Thaddeus D. |last12=Mansfield |first12=Megan |last13=Parmentier |first13=Vivien |last14=Thorngren |first14=Daniel |journal=Astronomy & Astrophysics |volume=668 |pages=L1 |doi=10.1051/0004-6361/202244533 |bibcode=2022A&A...668L...1J |s2cid=253708097 }}

File:Temperature vs Mass of known exoplanets.png

{{As of|2022}}, KELT-9b is the hottest known exoplanet, with dayside temperatures approaching {{convert|4600|K|C F|0|abbr=on}} — warmer than some K-type stars. Molecules on the day side are broken into their component atoms, so that normally sequestered refractory elements can exist as atomic species, including neutral oxygen,{{citation|arxiv=2112.12059|year=2022|title=High-resolution detection of neutral oxygen and non-LTE effects in the atmosphere of KELT-9b|doi=10.1038/s41550-021-01544-4|last1=Borsa|first1=Francesco|last2=Fossati|first2=Luca|last3=Koskinen|first3=Tommi|last4=Young|first4=Mitchell E.|last5=Shulyak|first5=Denis|journal=Nature Astronomy|volume=6|issue=2|pages=226–231|s2cid=245385802}} neutral and singly ionized atomic iron{{cite journal|arxiv=2004.11335|last1=Pino|first1=L.|last2=Désert|first2=J. M.|last3=Brogi|first3=M.|last4=Malavolta|first4=L.|last5=Wyttenbach|first5=A.|last6=Line|first6=M.|last7=Hoeijmakers|first7=J.|last8=Fossati|first8=L.|last9=Bonomo|first9=A. S.|last10=Nascimbeni|first10=V.|last11=Panwar|first11=V.|last12=Affer|first12=L.|last13=Benatti|first13=S.|last14=Biazzo|first14=K.|last15=Bignamini|first15=A.|last16=Borsa|first16=F.|last17=Carleo|first17=I.|last18=Claudi|first18=R.|last19=Cosentino|first19=R.|last20=Covino|first20=E.|last21=Damasso|first21=M.|last22=Desidera|first22=S.|last23=Giacobbe|first23=P.|last24=Harutyunyan|first24=A.|last25=Lanza|first25=A. F.|last26=Leto|first26=G.|last27=Maggio|first27=A.|last28=Maldonado|first28=J.|last29=Mancini|first29=L.|last30=Micela|first30=G.|title=Neutral Iron Emission Lines from the Day-side of KELT-9b -- the GAPS Programme with HARPS-N at TNG XX|journal=The Astrophysical Journal|year=2020|volume=894|issue=2|pages=L27|doi=10.3847/2041-8213/ab8c44|bibcode=2020ApJ...894L..27P|s2cid=216080480|display-authors=29 |doi-access=free }} (Fe and Fe⁺) and singly ionized titanium (Ti⁺), only to temporarily reform once they reach the cooler night side, which is indirectly confirmed by measured enhanced heat transfer efficiency of 0.3 between dayside and nightside, likely driven by the latent heat of dissociation and recombination of the molecular hydrogen.{{citation|arxiv=2208.04818|year=2022|title=The stable climate of KELT-9b|doi=10.1051/0004-6361/202243823 |last1=Jones |first1=K. |last2=Morris |first2=B. M. |last3=Demory |first3=B.-O. |last4=Heng |first4=K. |last5=Hooton |first5=M. J. |last6=Billot |first6=N. |last7=Ehrenreich |first7=D. |last8=Hoyer |first8=S. |last9=Simon |first9=A. E. |last10=Lendl |first10=M. |last11=Demangeon |first11=O. D. S. |last12=Sousa |first12=S. G. |last13=Bonfanti |first13=A. |last14=Wilson |first14=T. G. |last15=Salmon |first15=S. |last16=Csizmadia |first16=Sz. |last17=Parviainen |first17=H. |last18=Bruno |first18=G. |last19=Alibert |first19=Y. |last20=Alonso |first20=R. |last21=Anglada |first21=G. |last22=Bárczy |first22=T. |last23=Barrado |first23=D. |last24=Barros |first24=S. C. C. |last25=Baumjohann |first25=W. |last26=Beck |first26=M. |last27=Beck |first27=T. |last28=Benz |first28=W. |last29=Bonfils |first29=X. |last30=Brandeker |first30=A. |journal=Astronomy & Astrophysics |volume=666 |pages=A118 |bibcode=2022A&A...666A.118J |s2cid=251442580 |display-authors=1 }} Surprisingly, spectra taken in 2021 have unambiguously indicated a presence of metal oxides and hydrides in the planetary atmosphere,{{citation|arxiv=2101.00469|title=The Hubble WFC3 Emission Spectrum of the Extremely Hot Jupiter KELT-9b|year=2021|last1=Changeat|first1=Quentin|last2=Edwards|first2=Billy|journal=The Astrophysical Journal Letters|volume=907|issue=1|pages=L22|doi=10.3847/2041-8213/abd84f|bibcode=2021ApJ...907L..22C|s2cid=230435556 |doi-access=free }} although higher resolution spectra taken in 2021 have not found any molecular emissions from the planetary dayside.{{citation|arxiv=2108.08389|year=2021|title=Confirmation of Iron Emission Lines and Nondetection of TiO on the Dayside of KELT-9b with MAROON-X|doi=10.3847/2041-8213/ac30e1|last1=Kasper|first1=David|last2=Bean|first2=Jacob L.|last3=Line|first3=Michael R.|last4=Seifahrt|first4=Andreas|last5=Stürmer|first5=Julian|last6=Pino|first6=Lorenzo|last7=Désert|first7=Jean-Michel|last8=Brogi|first8=Matteo|journal=The Astrophysical Journal Letters|volume=921|issue=1|pages=L18|bibcode=2021ApJ...921L..18K|s2cid=239024467 |doi-access=free }}

The thermosphere layer of KELT-9b is expected to heat up to {{convert|10000|-|11000|K|C F|0|abbr=on}}, driven by ionization of heavy metals atoms like iron.{{citation|arxiv=2010.00997|title=A data-driven approach to constraining the atmospheric temperature structure of the ultra-hot Jupiter KELT-9b|year=2020|doi=10.1051/0004-6361/202039061|last1=Fossati|first1=L.|last2=Shulyak|first2=D.|last3=Sreejith|first3=A. G.|last4=Koskinen|first4=T.|last5=Young|first5=M. E.|last6=Cubillos|first6=P. E.|last7=Lara|first7=L. M.|last8=France|first8=K.|last9=Rengel|first9=M.|last10=Cauley|first10=P. W.|last11=Turner|first11=J. D.|last12=Wyttenbach|first12=A.|last13=Yan|first13=F.|journal=Astronomy & Astrophysics|volume=643|pages=A131|bibcode=2020A&A...643A.131F|s2cid=225127226}}

{{Planetary radius|base=Jupiter|Exoplanet=KELT-9b|radius=189}}{{Clear}}

• 51 Pegasi b, the first-discovered Hot Jupiter
• List of hottest exoplanets

{{reflist|refs=

{{cite journal | title=KELT-9 as an Eclipsing Double-lined Spectroscopic Binary: A Unique and Self-consistent Solution to the System | last1=Asnodkar | first1=Anusha Pai | last2=Wang | first2=Ji | last3=Gaudi | first3=B. Scott | last4=Cauley | first4=P. Wilson | last5=Eastman | first5=Jason D. | last6=Ilyin | first6=Ilya | last7=Strassmeier | first7=Klaus | last8=Beatty | first8=Thomas | display-authors=1 | journal=The Astronomical Journal | date=4 January 2022 | volume=163 | issue=2 | at=40 | arxiv=2110.15275 | doi=10.3847/1538-3881/ac32c7 | bibcode=2022AJ....163...40P | doi-access=free }}

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{{wikinews|Astronomers reveal discovery of the hottest gas giant exoplanet known yet}}

• {{SIMBAD link|id=HD+195689|label=HD 195689}} SIMBAD entry for HD 195689

{{2016 in space}}

{{Stars of Cygnus}}

Category:Cygnus (constellation)

Category:Transiting exoplanets

Category:Exoplanets discovered in 2016

Category:Exoplanets discovered by KELT

Category:Hot Jupiters