Letter to the Editor

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

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: b.jacobs@uva.nl
² INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
³ ASU School of Earth and Space Exploration, PO Box 871404 Tempe, AZ 85287-1404, USA
⁴ Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA
⁵ Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁶ Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43 221 00 Lund, Sweden
⁷ Department of Astronomy, University of Maryland, 4296 Stadium Drive, Astronomy Dept Room 1113, College Park, MD 20742, USA
⁸ Department of Physics (Atmospheric, Oceanic and Planetary Physics), Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
⁹ Trottier Institute for Research on Exoplanets (iREx), Département de physique, Université de Montréal, Complexe des Sciences, C. P. 6128, Succ. Centre-ville, Montreal, QC H3C 3J7, Canada

Received: 18 July 2022
Accepted: 13 November 2022

Abstract

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.