To constrain the formation history of an exoplanet, we need to know its chemical composition1,2,3. With an equilibrium temperature of about 4,050 kelvin4, the exoplanet KELT-9b (also known as HD 195689b) is an archetype of the class of ultrahot Jupiters that straddle the transition between stars and gas-giant exoplanets and are therefore useful for studying atmospheric chemistry. At these high temperatures, iron and several other transition metals are not sequestered in molecules or cloud particles and exist solely in their atomic forms5. However, despite being the most abundant transition metal in nature, iron has not hitherto been detected directly in an exoplanet because it is highly refractory. The high temperatures of KELT-9b imply that its atmosphere is a tightly constrained chemical system that is expected to be nearly in chemical equilibrium5 and cloud-free6,7, and it has been predicted that spectral lines of iron should be detectable in the visible range of wavelengths5. Here we report observations of neutral and singly ionized atomic iron (Fe and Fe+) and singly ionized atomic titanium (Ti+) in the atmosphere of KELT-9b. We identify these species using cross-correlation analysis8 of high-resolution spectra obtained as the exoplanet passed in front of its host star. Similar detections of metals in other ultrahot Jupiters will provide constraints for planetary formation theories.

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This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (projects Four Aces and EXOKLEIN with grant agreement numbers 724427 and 771620, respectively). This work was carried out in the framework of the PlanetS National Centre of Competence in Research (NCCR) supported by the Swiss National Science Foundation (SNSF).

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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  1. Observatoire astronomique de l’Université de Genève, Versoix, Switzerland

    • H. Jens Hoeijmakers
    • , David Ehrenreich
    • , Romain Allart
    • , Aurélien Wyttenbach
    •  & Lorenzo Pino
  2. University of Bern, Center for Space and Habitability, Bern, Switzerland

    • H. Jens Hoeijmakers
    • , Kevin Heng
    • , Daniel Kitzmann
    • , Simon L. Grimm
    • , Russell Deitrick
    •  & Maria Oreshenko
  3. University of Cambridge, Cavendish Astrophysics, Cambridge, UK

    • Paul B. Rimmer
  4. MRC Laboratory of Molecular Biology, Cambridge, UK

    • Paul B. Rimmer
  5. INAF FGG, Telescopio Nazionale Galileo, Breña Baja, Spain

    • Emilio Molinari
    •  & Luca Di Fabrizio
  6. INAF Osservatorio Astronomici di Cagliari, Selargius, Italy

    • Emilio Molinari


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H.J.H. led the data reduction and analysis, designed the cross-correlation templates, co-wrote the manuscript, co-led the scientific vision and coordination, sourced database information for opacity calculations and made all of the plots in Fig. 3. D.E. is the principal investigator of the HARPS-N dataset (A35DDT4) and co-wrote the manuscript. K.H. led the scientific vision, coordination and interpretation, and the writing of the manuscript. D.K. performed chemical-equilibrium calculations using the FastChem computer code and made the corresponding plot in Fig. 1. S.L.G. sourced database information for and performed opacity calculations using HELIOS-K computer code. R.A. performed the telluric subtraction using Molecfit and confirmed detections of Fe, Fe+ and Ti+ by repeating the cross-correlation analysis with binary masks. R.D. compiled cross-sections for spectral lines and continuum and made the corresponding plot in Fig. 1. A.W. led the data acquisition and observation, performed the extraction of the spectra, and confirmed the Doppler-shadow measurement and performed its removal. M.O. made the schematic plot in Fig. 2. L.P. co-designed the cross-correlation templates and performed additional checks on the outcome. P.B.R. performed chemical-kinetics calculations using ARGO computer code and made the corresponding plot in Fig. 1. E.M. provided a night of Director’s Discretionary Time to make the project possible. L.D.F. performed the TNG/HARPS-N observations.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Kevin Heng.

Extended data figures and tables

  1. Extended Data Fig. 1 Telluric water-absorption correction.

    Cross-correlation analysis performed on the in-transit spectrum with a telluric water-absorption template spectrum (at 296 K). The blue and grey curves are the transit depths with and without the telluric correction, respectively. The data are shifted to the rest frame of the star, such that the signal dominated by telluric water absorption occurs near +25 km s−1 (corresponding to the barycentric Earth radial velocity (BERV) systemic velocity), as indicated by the dashed vertical line.

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