Thermal emission spectra of the ultra-hot Jupiter WASP-33 b

¹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
² Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, PR China
⁴ CAS Key Laboratory of Planetary Science, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, PR China
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁶ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁷ Deep Space Exploration Laboratory, Hefei, Anhui 230026, China
⁸ Department of Science Research, Beijing Planetarium, Beijing 100044, PR China
⁹ State Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
¹⁰ Max Planck Institute for Astronomy, Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

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Received: 10 November 2025
Accepted: 28 January 2026

Abstract

Observations of exoplanetary atmospheres provide critical insights into their chemical composition, formation, and evolution history. Ultra-hot Jupiters serve as excellent targets for atmospheric characterization; studies of these planets may yield key understanding of gas giants’ formation and evolution history. We present a thermal emission study of WASP-33 b’s dayside atmosphere, based on two secondary eclipse observations with CFHT/WIRCam in two specific narrow band filters, namely the CO and CH4_on filters, and archival data with HST/WFC3 and Spitzer. Stellar pulsations of the host star induce some quasi-periodic photometric variations, particularly in the CH4_on band, which are modeled and corrected in the high-precision differential light curves. An eclipse depth of 1565.2_−237.5^+228.6 ppm and 914.3_−57.0^+56.1 ppm is determined for the CO and CH4_on bands, respectively. Combined with HST/WFC3 and Spitzer data, our joint retrieval of WASP-33 b’s dayside atmosphere reveals a high metallicity ([Fe/H] = 1.52_−0.52^+0.35), high C/O ratio (C/O = 0.78_−0.04^+0.03), and a thermal inversion layer, suggesting a formation history involving metal-rich gas accretion. We confirm the presence of the molecules H₂O, H⁺ and CO, and report a tentative detection of TiO in the dayside atmosphere of WASP-33 b. Future higher precision observations with JWST may provide better understanding of constraints on the chemical abundances of oxygen and refractory element abundances to better constrain WASP-33 b’s formation and evolutionary pathway.