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 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[1][7] 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.[8][9]
KELT-9b is a relatively large giant planet at about 2.8 times the massofJupiter; 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.[9]
The outer boundary of its atmosphere nearly reaches its Roche lobe, implying that the planet is experiencing rapid atmospheric escape[10] driven by the extreme amount of radiation it receives from its host star.[9][8] In 2020, atmospheric loss rate was measured to be equal to 18 - 68 Earth masses per billion years.[11]
The planet's elemental abundances remain largely unknown as of 2022, but a low carbon-to-oxygen ratio is strongly suspected.[12]
This graph shows the average temperature and mass relative to Jupiter (Mj) of known exoplanets as of 2022
As of 2022[update], 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.[1][4] 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,[13] neutral and singly ionized atomic iron[14] (Fe and Fe+) and singly ionized titanium (Ti+),[15] only to temporarily reform once they reach the cooler night side,[4] which is indirectly confirmed by measured enhanced heat transfer efficiency of 0.3 between dayside and nightside, likely diven by the latent heat of dissociation and recombination of the molecular hydrogen.[3] Surprisingly, spectra taken in 2021 have unambiguously indicated a presence of metal oxides and hydrides in the planetary atmosphere,[16] although higher resolution spectra taken in 2021 have not found any molecular emissions from the planetary dayside.[17]
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.[18]
^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.
^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.
^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. S2CID216641961.
^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, S2CID253708097
^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, S2CID245385802
^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, S2CID239024467
^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, S2CID225127226
