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.
The authors declare no competing financial interests.
Reviewer Information Nature thanks D. Deming and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Left, middle and right columns correspond to the three nights when KELT-9 was observed during transit. Top row, spectroscopic data (orbital phase); middle row, the derived model; bottom row, the residuals. In each panel, the vertical blue lines denote the width of the convolution kernel (that is, v sin I*), and the horizontal blue lines show the duration of the transit. Time increases vertically for each panel. The apparent extension of the Doppler shadow before ingress on ut 2014 October 05 is an artefact of uneven and widely spaced sampling in time. The greyscale shows the fractional variation in the spectroscopic signal from the null hypothesis of no shadow due to a transiting planet. Darker regions indicate the Doppler shadow as the planet crosses the face of the host star. Note the transit is nearly coincident with the projected stellar spin axis, implying a nearly polar orbit for the planet.
Crosses represent the measured fluxes, with vertical error bars representing the uncertainties in the measurements as quantified by the standard deviation and the horizontal error bars representing the width of the bandpass. The blue dots are the predicted passband-integrated fluxes of the best-fit theoretical spectral energy distribution (SED) corresponding to our observed photometric bands. The black curve represents the best-fit Kurucz stellar atmosphere44. Here λ is the wavelength and Fλ the monochromatic flux (flux at wavelength λ).
The red cross represents the system parameters from the initial SED fit, the blue cross represents the final system parameters from the global fit. The black curve represents the theoretical evolutionary track for a star with the mass and metallicity of KELT-9, and the grey swath bounded by dashed lines represents the uncertainty (standard deviation) on that track based on the uncertainties in mass and metallicity. Nominal ages in billions of years are shown as blue dots. When KELT-9 evolves to the base of the giant branch in 200–300 Myr, it will encroach upon the orbit of KELT-9b. The fate of the system at that point is highly uncertain20,21,55,56,57. Here Teff is the stellar effective temperature and g* is the surface gravity.
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Gaudi, B., Stassun, K., Collins, K. et al. A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host. Nature 546, 514–518 (2017). https://doi.org/10.1038/nature22392
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