KELT-9b

KELT-9b
Artist's impression of KELT-9b and its parent star
Discovery
Discovered byB. Scott Gaudi, et al.
Discovery siteKELT-North, Arizona
Discovery dateMay 2016 (announced)
June 2017 (published)
Transit
Designations
HD 195689 b
Orbital characteristics
0.03368±0.00078 AU
Eccentricity0
1.4811235±0.0000011 d
Inclination86.79°±0.25°
Semi-amplitude293±32 m/s
StarHD 195689 A
Physical characteristics
1.82±0.03 RJ
Mass2.22±0.58 MJ
Mean density
0.491+0.072
−0.066 g/cm3
Albedo<0.14
Temperature4,579±120 K (dayside)
2,400+400
−300 K (nightside)

KELT-9b, also designated HD 195689 b, is an exoplanet and ultra-hot Jupiter in a polar orbit around the late B-type/early A-type star KELT-9, located about 670 light-years from Earth. Detected using the Kilodegree Extremely Little Telescope, the discovery of KELT-9b was announced in 2016. As of October 2026, it is the hottest known exoplanet.

Host star

The host star, KELT-9, also designated HD 195689, is 2.3 times larger and 2.4 times more massive than the sun. The surface temperature is 10,170 K (9,897 °C; 17,846 °F), 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 7,430 K (7,157 °C; 12,914 °F); 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.

Physical properties

Exoplanet KELT-9b orbits host star KELT-9

KELT-9b is a relatively large giant planet at about 2.2 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. 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.

The planet's elemental abundances remain largely unknown as of 2022, but a low carbon-to-oxygen ratio is strongly suspected.

This graph shows the average temperature and mass relative to Jupiter (Mj) of known exoplanets as of 2022

As of 2026, KELT-9b is the hottest known exoplanet, with dayside temperatures approaching 4,600 K (4,327 °C; 7,820 °F) — 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, neutral and singly ionized atomic iron (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. Surprisingly, spectra taken in 2021 have unambiguously indicated a presence of metal oxides and hydrides in the planetary atmosphere, although higher resolution spectra taken in 2021 have not found any molecular emissions from the planetary dayside. In 2023, singly-ionized terbium was observed on the planet, making it the first detection of terbium in an exoplanet's atmosphere.

The thermosphere layer of KELT-9b is expected to heat up to 10,000–11,000 K (9,727–10,727 °C; 17,540–19,340 °F), driven by ionization of heavy metals atoms like iron.

In June 2026 scientists found a self-absorption phenomena of Hα in the upper atmosphere KELT-9b. Due to this, it is suspected that the planet is losing 1013 grams per second and has a vertical outflow wind speed of ~5 km/s. Another 2026 study potentially detected magnetic field for the planet, via observation of the Zeeman effect with the SPIRou spectrograph.

Size comparison
Jupiter KELT-9b
Jupiter Exoplanet

See also

References

  1. 1 2 3 4 Gaudi, B. Scott; et al. (5 June 2017). "A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host" (pdf). Nature. 546 (7659): 514–518. arXiv:1706.06723. Bibcode:2017Natur.546..514G. doi:10.1038/nature22392. ISSN 1476-4687. PMID 28582774. S2CID 205256410. Retrieved 2017-06-06.
  2. 1 2 Collins, Karen A.; Stassun, Keivan; Gaudi, B. Scott; Beatty, Thomas G.; Zhou, George; Latham, David W.; Bieryla, Allyson; Eastman, Jason D.; Siverd, Robert; Crepp, Justin R.; Pepper, Joshua (2016). "KELT-9b: A Case Study in Dynamical Planet Ingestion by a Hot Host Star". American Astronomical Society. 47: 204.03. Bibcode:2016DDA....4720403C.
  3. 1 2 Borsa, F.; Rainer, M.; et al. (November 2019). "The GAPS Programme with HARPS-N at TNG: XIX. Atmospheric Rossiter-McLaughlin effect and improved parameters of KELT-9b". Astronomy & Astrophysics. 631: A34. arXiv:1907.10078. Bibcode:2019A&A...631A..34B. doi:10.1051/0004-6361/201935718. ISSN 0004-6361.
  4. 1 2 3 4 Zhang, Yapeng; Wardenier, Joost P.; Householder, Aaron; Komacek, Thaddeus D.; Kesseli, Aurora; Dai, Fei; Howard, Andrew W.; Inglis, Julie; Knutson, Heather A.; Mawet, Dimitri; Pino, Lorenzo; Wallack, Nicole; Xuan, Jerry W.; Carmichael, Theron W.; Huber, Daniel (2026-02-01). "Extreme winds on the emerging dayside of an ultrahot Jupiter". The Astrophysical Journal Letters. 997 (2): L40. arXiv:2601.20849. Bibcode:2026ApJ...997L..40Z. doi:10.3847/2041-8213/ae36a1. ISSN 2041-8205.
  5. 1 2 Jones, K.; et al. (2022), "The stable climate of KELT-9b", Astronomy & Astrophysics, 666: A118, arXiv:2208.04818, Bibcode:2022A&A...666A.118J, doi:10.1051/0004-6361/202243823, S2CID 251442580
  6. 1 2 3 4 Brennan, Pat; Cofield, Calia (24 January 2020). "For Hottest Planet, a Major Meltdown, Study Shows". NASA. Retrieved 24 January 2020.
  7. ↑ Tribur, Melissa (2017-06-05). "Meet KELT-9b, the Hottest Exoplanet Ever Discovered". Eos. Retrieved 2022-10-28.
  8. ↑ Jensen, K. S. (1981). "Spectral Classification in the MK System of 167 Northern HD Stars". Astronomy and Astrophysics Supplement. 45: 455. Bibcode:1981A&AS...45..455J.
  9. 1 2 KELT-9 b Exoplanet Exploration Program 2017
  10. 1 2 3 NASA JPL, Pasadena CA (5 June 2017) Astronomers Find Planet Hotter Than Most Stars
  11. ↑ Yan, Fei; Henning, Thomas (2 July 2018). "An extended hydrogen envelope of the extremely hot giant exoplanet KELT-9b". Nature Astronomy. 2 (9): 714–718. arXiv:1807.00869. Bibcode:2018NatAs...2..714Y. doi:10.1038/s41550-018-0503-3. ISSN 2397-3366. S2CID 119405172. Retrieved 18 August 2018.
  12. ↑ Wyttenbach, A.; Mollière, P.; Ehrenreich, D.; Cegla, H. M.; Bourrier, V.; Lovis, C.; Pino, L.; Allart, R.; Seidel, J. V.; Hoeijmakers, H. J.; Nielsen, L. D.; Lavie, B.; Pepe, F.; Bonfils, X.; Snellen, I. A. G. (2020). "Mass loss rate and local thermodynamic state of KELT-9 b thermosphere from the hydrogen Balmer series". Astronomy & Astrophysics. 638: A87. arXiv:2004.13733. Bibcode:2020A&A...638A..87W. doi:10.1051/0004-6361/201937316. S2CID 216641961.
  13. ↑ Jacobs, Bob; Désert, Jean-Michel; Pino, Lorenzo; Line, Michael R.; Bean, Jacob L.; Khorshid, Niloofar; Schlawin, Everett; Arcangeli, Jacob; Barat, Saugata; Jens Hoeijmakers, H.; Komacek, Thaddeus D.; Mansfield, Megan; Parmentier, Vivien; Thorngren, Daniel (2022), "A strong H− opacity signal in the near-infrared emission spectrum of the ultra-hot Jupiter KELT-9b", Astronomy & Astrophysics, 668: L1, arXiv:2211.10297, Bibcode:2022A&A...668L...1J, doi:10.1051/0004-6361/202244533, S2CID 253708097
  14. ↑ Borsa, Francesco; Fossati, Luca; Koskinen, Tommi; Young, Mitchell E.; Shulyak, Denis (2022), "High-resolution detection of neutral oxygen and non-LTE effects in the atmosphere of KELT-9b", Nature Astronomy, 6 (2): 226–231, arXiv:2112.12059, doi:10.1038/s41550-021-01544-4, S2CID 245385802
  15. ↑ Pino, L.; Désert, J. M.; Brogi, M.; Malavolta, L.; Wyttenbach, A.; Line, M.; Hoeijmakers, J.; Fossati, L.; Bonomo, A. S.; Nascimbeni, V.; Panwar, V.; Affer, L.; Benatti, S.; Biazzo, K.; Bignamini, A.; Borsa, F.; Carleo, I.; Claudi, R.; Cosentino, R.; Covino, E.; Damasso, M.; Desidera, S.; Giacobbe, P.; Harutyunyan, A.; Lanza, A. F.; Leto, G.; Maggio, A.; Maldonado, J.; Mancini, L.; et al. (2020). "Neutral Iron Emission Lines from the Day-side of KELT-9b -- the GAPS Programme with HARPS-N at TNG XX". The Astrophysical Journal. 894 (2): L27. arXiv:2004.11335. Bibcode:2020ApJ...894L..27P. doi:10.3847/2041-8213/ab8c44. S2CID 216080480.
  16. ↑ Hoeijmakers, H. Jens; Ehrenreich, David; Heng, Kevin; Kitzmann, Daniel; Grimm, Simon L.; Allart, Romain; Deitrick, Russell; Wyttenbach, Aurélien; Oreshenko, Maria; Pino, Lorenzo; Rimmer, Paul B.; Molinari, Emilio; Di Fabrizio, Luca (15 August 2018). "Atomic iron and titanium in the atmosphere of the exoplanet KELT-9b". Nature. 560 (7719): 453–455. arXiv:1808.05653. Bibcode:2018Natur.560..453H. doi:10.1038/s41586-018-0401-y. ISSN 1476-4687. PMC 6322651. PMID 30111838.
  17. ↑ Changeat, Quentin; Edwards, Billy (2021), "The Hubble WFC3 Emission Spectrum of the Extremely Hot Jupiter KELT-9b", The Astrophysical Journal Letters, 907 (1): L22, arXiv:2101.00469, Bibcode:2021ApJ...907L..22C, doi:10.3847/2041-8213/abd84f, S2CID 230435556
  18. ↑ Kasper, David; Bean, Jacob L.; Line, Michael R.; Seifahrt, Andreas; Stürmer, Julian; Pino, Lorenzo; Désert, Jean-Michel; Brogi, Matteo (2021), "Confirmation of Iron Emission Lines and Nondetection of TiO on the Dayside of KELT-9b with MAROON-X", The Astrophysical Journal Letters, 921 (1): L18, arXiv:2108.08389, Bibcode:2021ApJ...921L..18K, doi:10.3847/2041-8213/ac30e1, S2CID 239024467
  19. ↑ "Terbium Detected in Atmosphere of Ultrahot Jupiter KELT-9b". sci.news. April 2023.
  20. ↑ Borsato, N. W.; Hoeijmakers, H. J.; Prinoth, B.; Thorsbro, B.; Forsberg, R.; Kitzmann, D.; Jones, K.; Heng, K. (May 2023). "The Mantis Network: III. Expanding the limits of chemical searches within ultra-hot Jupiters: New detections of Ca I, V I, Ti I, Cr I, Ni I, Sr II, Ba II, and Tb II in KELT-9 b". Astronomy & Astrophysics. 673: A158. arXiv:2304.04285. Bibcode:2023A&A...673A.158B. doi:10.1051/0004-6361/202245121. ISSN 0004-6361.
  21. ↑ Fossati, L.; Shulyak, D.; Sreejith, A. G.; Koskinen, T.; Young, M. E.; Cubillos, P. E.; Lara, L. M.; France, K.; Rengel, M.; Cauley, P. W.; Turner, J. D.; Wyttenbach, A.; Yan, F. (2020), "A data-driven approach to constraining the atmospheric temperature structure of the ultra-hot Jupiter KELT-9b", Astronomy & Astrophysics, 643: A131, arXiv:2010.00997, Bibcode:2020A&A...643A.131F, doi:10.1051/0004-6361/202039061, S2CID 225127226
  22. ↑ Zhang, Yapeng (June 24, 2026). "Hydrogen Airglow from an Escaping Ultrahot Jupiter Atmosphere". The Astrophysical Journal Letters. 1005 (1): L29. arXiv:2606.23899. Bibcode:2026ApJ..1005L..29Z. doi:10.3847/2041-8213/ae7be6.
  23. ↑ Lavail, Alexis; Debras, Florian; David Rogé Royo; Arturo López Ariste; Borsato, Nicholas W.; Donati, Jean-François; Petit, Pascal; Alvarado Gómez, Julian D.; Jouve, Laurène; Moutou, Claire; Carmona, Andres; Klein, Baptiste; Seidel, Julia V.; Arnold, Luc; Artigau, Etienne; Baruteau, Clément; Chabrol, Estelle; Cloutier, Ryan; Cook, Neil J.; Cowan, Nicolas B.; Delfosse, Xavier; Heidari, Neda; Martioli, Eder; Masson, Adrien; Wardenier, Joost P.; Yariv, Vincent (2026). "SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: tentative detection of the planet magnetic field". arXiv:2610.05951 [astro-ph.EP].