The amount of ultraviolet irradiation and ablation experienced by a planet depends strongly on the temperature of its host star. Of the thousands of extrasolar planets now known, only six have been found that transit hot, A-type stars (with temperatures of 7,300–10,000 kelvin), and no planets are known to transit the even hotter B-type stars. For example, WASP-33 is an A-type star with a temperature of about 7,430 kelvin, which hosts the hottest known transiting planet, WASP-33b (ref. 1); the planet is itself as hot as a red dwarf star of type M (ref. 2). WASP-33b displays a large heat differential between its dayside and nightside2, and is highly inflated–traits that have been linked to high insolation3,4. However, even at the temperature of its dayside, its atmosphere probably resembles the molecule-dominated atmospheres of other planets and, given the level of ultraviolet irradiation it experiences, its atmosphere is unlikely to be substantially ablated over the lifetime of its star. Here we report observations of the bright star HD 195689 (also known as KELT-9), which reveal a close-in (orbital period of about 1.48 days) transiting giant planet, KELT-9b. At approximately 10,170 kelvin, the host star is at the dividing line between stars of type A and B, and we measure the dayside temperature of KELT-9b to be about 4,600 kelvin. This is as hot as stars of stellar type K4 (ref. 5). The molecules in K stars are entirely dissociated, and so the primary sources of opacity in the dayside atmosphere of KELT-9b are probably atomic metals. Furthermore, KELT-9b receives 700 times more extreme-ultraviolet radiation (that is, with wavelengths shorter than 91.2 nanometres) than WASP-33b, leading to a predicted range of mass-loss rates that could leave the planet largely stripped of its envelope during the main-sequence lifetime of the host star6.

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This research was made possible by the KELT survey, the KELT Follow-Up Network, and support from The Ohio State University, Vanderbilt University and Lehigh University. Work by B.S.G. and D.J.S. was partially supported by NSF CAREER grant AST-1056524. K.G.S. and K.A.C. acknowledge partial support from NSF PAARE grant AST-1358862. B.S.G. acknowledges support by the Jet Propulsion Laboratory, operated by the California Institute of Technology, and the Exoplanet Exploration Program of the National Aeronautics and Space Administration (NASA). B.J.F. notes that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant no. 2014184874. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors(s) and do not necessarily reflect the views of the National Science Foundation. Work performed by J.E.R. was supported by the Harvard Future Faculty Leaders Postdoctoral fellowship. K.K.M. acknowledges the purchase of SDSS filters for Whitin Observatory by the Theodore Dunham Jr Grant of the Fund for Astronomical Research. N.N. acknowledges support by the Japan Society for Promotion of Science (JSPS) KAKENHI Grant Number JP25247026. We acknowledge observations by M. Kunitomo, R. Hasegawa, B. Sato, H. Harakawa, T. Hirano and H. Izumiura on the Okayama 188 cm telescope (HIDES observations) and N. Kusakabe, M. Onitsuka and T. Ryu for MuSCAT observations. The NIRC2 AO data in this work were obtained at the W.M.Keck Observatory, which was financed by the W.M. Keck Foundation and is operated as a scientific partnership between the California Institute of Technology, the University of California and NASA. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work has made use of NASA’s Astrophysics Data System, the Exoplanets Data Explorer at exoplanets.org, the Extrasolar Planet Encyclopedia at exoplanet.eu, the SIMBAD database operated at CDS, Strasbourg, France, and the VizieR catalogue access tool, CDS, Strasbourg, France. This publication makes use of data products from the Widefield Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles; the Jet Propulsion Laboratory/California Institute of Technology, which is funded by NASA; the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA; and the American Association of Variable Star Observers (AAVSO) Photometric All-Sky Survey (APASS), whose funding is provided by the Robert Martin Ayers Sciences Fund and the AAVSO Endowment (https://www.aavso.org/aavso-photometric-all-sky-survey-data-release-1). We acknowledge input from T. Barman, J. Fortney, M. Marley and K. Zanhle.

Author information


  1. Department of Astronomy, The Ohio State University, Columbus, Ohio 43210, USA

    • B. Scott Gaudi
    • , Daniel J. Stevens
    • , Marshall C. Johnson
    • , Matthew Penny
    • , Andrew Gould
    •  & Richard W. Pogge
  2. Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, Nashville, Tennessee 37235, USA

    • Keivan G. Stassun
    • , Karen A. Collins
    •  & Michael B. Lund
  3. Department of Physics, Fisk University, 1000 17th Avenue North, Nashville, Tennessee 37208, USA

    • Keivan G. Stassun
  4. Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, 525 Davey Laboratory, University Park, Pennsylvania 16802, USA

    • Thomas G. Beatty
  5. Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, Pennsylvania 16802, USA

    • Thomas G. Beatty
  6. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

    • George Zhou
    • , David W. Latham
    • , Allyson Bieryla
    • , Jason D. Eastman
    • , Joseph E. Rodriguez
    • , Gilbert A. Esquerdo
    •  & Howard Relles
  7. Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, Suite 102, Santa Barbara, California 93117, USA

    • Robert J. Siverd
  8. Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, Indiana 46556, USA

    • Justin R. Crepp
    •  & Erica J. Gonzales
  9. Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 21S00 Copenhagen, Denmark

    • Lars A. Buchhave
  10. Centre for Star and Planet Formation, Geological Museum, Øster Voldgade 5, 1350 Copenhagen, Denmark

    • Lars A. Buchhave
  11. Department of Physics, Lehigh University, 16 Memorial Drive East, Bethlehem, Pennsylvania 18015, USA

    • Joshua Pepper
    •  & Jonathan Labadie-Bartz
  12. NASA Ames Research Center, M/S 244-30, Moffett Field, California 94035, USA

    • Knicole D. Colon
  13. Bay Area Environmental Research Institute, 625 2nd Street Ste. 209, Petaluma, California 94952, USA

    • Knicole D. Colon
  14. Department of Physics and Astronomy, Swarthmore College, Swarthmore, Pennsylvania 19081, USA

    • Eric L. N. Jensen
  15. Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno, Via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy

    • Valerio Bozza
    • , Sebastiano Calchi Novati
    •  & Gaetano Scarpetta
  16. Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, 80126 Napoli, Italy

    • Valerio Bozza
  17. IPAC, Mail Code 100-22, Caltech, 1200 East California Boulevard, Pasadena, California 91125, USA

    • Sebastiano Calchi Novati
    •  & Gaetano Scarpetta
  18. Istituto Internazionale per gli Alti Studi Scientifici (IIASS), Via G. Pellegrino 19, 84019 Vietri sul Mare (SA), Italy

    • Giuseppe D’Ago
  19. INAF-Observatory of Capodimonte, Salita Moiariello 16, 80131 Naples, Italy

    • Giuseppe D’Ago
  20. Department of Physics and Astronomy, Brigham Young University, Provo, Utah 84602, USA

    • Mary T. Dumont
    • , Clement Gaillard
    • , Kyle Matt
    • , Denice C. Stephens
    •  & Michael D. Joner
  21. Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, California 95064, USA

    • Mary T. Dumont
  22. Department of Physics and Astronomy, University of Wyoming, 1000 East University, Laramie, Wyoming 82071, USA

    • Tyler Ellis
    • , Hannah Jang-Condell
    • , David H. Kasper
    •  & Rex R. Yeigh
  23. Department of Physics and Astronomy, Louisiana State University, 202 Nicholson Hall, Baton Rouge, Louisiana 70803, USA

    • Tyler Ellis
  24. Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, NINS, Asakuchi, Okayama 719-0232, Japan

    • Akihiko Fukui
  25. Atalaia Group and Crow Observatory, Portalegre, Portugal

    • Joao Gregorio
  26. National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

    • Ayaka Ito
    •  & Norio Narita
  27. Graduate School of Science and Engineering, Hosei University, 3-7-2 Kajino-cho, Koganeishi, Tokyo 184-8584, Japan

    • Ayaka Ito
  28. Department of Physics and Astronomy, University of Louisville, Louisville, Kentucky 40292, USA

    • John F. Kielkopf
  29. Spot Observatory, Nashville, Tennessee 37206, USA

    • Mark Manner
  30. Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

    • Norio Narita
  31. Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

    • Norio Narita
  32. Department of Physics, Westminster College, New Wilmington, Pennsylvania 16172, USA

    • Thomas E. Oberst
  33. Department of Physical Sciences, Kutztown University, Kutztown, Pennsylvania 19530, USA

    • Phillip A. Reed
  34. Società Astronomica Lunae, Castelnuovo Magra 19030, Italy

    • Roberto Zambelli
  35. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839, USA

    • B. J. Fulton
    •  & Andrew W. Howard
  36. Astronomy Department, University of Washington, Box 351580, Seattle, Washington 98195, USA

    • David J. James
  37. Observatoire Astronomique de l’Université de Genève, 51 Chemin des Maillettes, 1290 Versoix, Switzerland

    • Daniel Bayliss
  38. ICO, Adelaide, South Australia, Australia

    • Ivan A. Curtis
  39. George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, and Department of Physics and Astronomy, Texas A & M University, College Station, Texas 77843-4242, USA

    • D. L. DePoy
    •  & Jennifer L. Marshall
  40. Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany

    • Andrew Gould
  41. South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa

    • Rudolf B. Kuhn
  42. Wellesley College, 106 Central Street, Wellesley, Massachusetts 02481, USA

    • Kim K. McLeod
  43. Hazelwood Observatory, Churchill, Victoria, Australia

    • Christopher Stockdale
  44. Perth Exoplanet Survey Telescope, Perth, Western Australia, Australia

    • T. G. Tan
  45. Winer Observatory, Sonoita, Arizona 85637, USA

    • Mark Trueblood
    •  & Patricia Trueblood


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B.S.G. led the process from initial candidate selection to final planet confirmation. B.S.G. and K.G.S. wrote the majority of the main manuscript and contributed to the analysis of the results. K.A.C. principally coordinated the assemblage of the final results and production of the Methods. T.G.B. and G.Z. led the Doppler tomographic analysis, and also contributed to the final modelling and interpretation. J.D.E., R.J.S., D.J.S., L.A.B., J.P., J.E.R., K.D.C., M.C.J. and M.P. all provided critical insight, analysis or interpretation of the system. G.D., V.B., S.C.N., M.T.D., T.E., C.G., H.J.-C., D.H.K., A.F., J.G., A.I., J.F.K., M.M., K.M., N.N., T.E.O., P.A.R., G.S., D.C.S., R.R.Y., R.Z., B.J.F. and A.H. all provided photometric or radial velocity data that were important for the interpretation of the system. J.C. and E.J.G. provided the observations and analysis of the Keck AO data. E.L.N.J., D.J.J., D.B., I.A.C., D.L.D., G.A.E., A.G., M.D.J., R.B.K., J.L.-B., M.B.L., J.M., K.K.M., R.W.P., H.R., D.C.S., C.S., T.G.T., M.T. and P.T. have all been essential for the initiation and successful operation of the KELT-North and KELT-South surveys. All of the authors have read the manuscript and concur with the conclusions therein.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to B. Scott Gaudi.

Reviewer Information Nature thanks D. Deming and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data