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

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


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.

Original languageEnglish (US)
Article numberL1
JournalAstronomy and Astrophysics
StatePublished - Dec 1 2022


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

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'A strong H-opacity signal in the near-infrared emission spectrum of the ultra-hot Jupiter KELT-9b'. Together they form a unique fingerprint.

Cite this