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Lego Figure holding a lego telescope. Photo

Jens Hoeijmakers


Lego Figure holding a lego telescope. Photo

A strong H-opacity signal in the near-infrared emission spectrum of the ultra-hot Jupiter KELT-9b


  • B. Jacobs
  • J. M. Désert
  • L. Pino
  • M. R. Line
  • J. L. Bean
  • N. Khorshid
  • E. Schlawin
  • J. Arcangeli
  • S. Barat
  • H. J. Hoeijmakers
  • T. D. Komacek
  • M. Mansfield
  • V. Parmentier
  • D. Thorngren

Summary, in English

We present the analysis of a spectroscopic secondary eclipse of the hottest transiting exoplanet detected to date, KELT-9b, obtained with the Wide Field Camera 3 aboard the Hubble Space Telescope. We complement these data with literature information on stellar pulsations and Spitzer/Infrared Array Camera and Transiting Exoplanet Survey Satellite eclipse depths of this target to obtain a broadband thermal emission spectrum. Our extracted spectrum exhibits a clear turnoff at 1.4 μm. This points to H- bound-free opacities shaping the spectrum. To interpret the spectrum, we perform grid retrievals of self-consistent 1D equilibrium chemistry forward models, varying the composition and energy budget. The model with solar metallicity and C/O ratio provides a poor fit because the H- signal is stronger than expected, requiring an excess of electrons. This pushes our retrievals toward high atmospheric metallicities ([M/H] = 1.98-0.21+0.19) and a C/O ratio that is subsolar by 2.4σ. We question the viability of forming such a high-metallicity planet, and therefore provide other scenarios to increase the electron density in this atmosphere. We also look at an alternative model in which we quench TiO and VO. This fit results in an atmosphere with a slightly subsolar metallicity and subsolar C/O ratio ([M/H] = -0.22-0.13+0.17, log (C/O) = -0.34-0.34+0.19). However, the required TiO abundances are disputed by recent high-resolution measurements of the same planet.


  • Lund Observatory - Undergoing reorganization
  • Astrophysics

Publishing year





Astronomy and Astrophysics



Document type

Journal article


EDP Sciences


  • Astronomy, Astrophysics and Cosmology


  • Planets and satellites: atmospheres
  • Planets and satellites: gaseous planets




  • ISSN: 0004-6361