Enriched volatiles and refractories but deficient titanium on the day-side atmosphere of WASP-121b revealed by JWST/NIRISS

¹ Observatoire astronomique de l’Université de Genève, 51 chemin Pegasi 1290 Versoix, Switzerland
² Trottier Institute for Research on Exoplanets, Department of Physics, Université de Montréal, 1375 Avenue Thérèse-Lavoie-Roux, Montréal, QC H2V 0B3, Canada
³ Trottier Space Institute at McGill, 3550 rue University, Montréal, QC H3A 2A7, Canada
⁴ Department of Earth & Planetary Sciences, McGill University, 3450 rue University, Montreal, QC H3A OE8, Canada
⁵ Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA
⁶ Department of Astronomy, University of Michigan, 1085 S. University Ave., Ann Arbor, MI 48109, USA
⁷ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
⁸ Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
⁹ Observatoire du Mont-Mégantic, Université de Montréal, C.P. 6128, Succ. Centre-ville, Montréal, QC H3C 3J7, Canada
¹⁰ Department of Physics and Astronomy, University of Waterloo, 200 University W, Waterloo, ON N2L 3G1, Canada
¹¹ Department of Astronomy & Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA, 16802, USA
¹² Center for Exoplanets and Habitable Worlds, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
¹³ NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada
¹⁴ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
¹⁵ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
¹⁶ Department of Astronomy and Carl Sagan Institute, Cornell University, Ithaca, NY 14850, USA
¹⁷ Department of Astronomy & Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA
¹⁸ Department of Physics & Astronomy, Bishop’s University, 2600 Rue College, Sherbrooke, QC J1M 1Z7, Canada
¹⁹ Department of Physics, University of Oxford, Parks Rd, Oxford OX1 3PU, UK
²⁰ Department of Astronomy and Carl Sagan Institute, Cornell University, Ithaca, NY 14853, USA

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Received: 25 August 2025
Accepted: 24 November 2025

Abstract

Context. With day-side temperatures elevated enough for all atmospheric constituents to be present in gas form, ultra-hot Jupiters offer a unique opportunity to probe the composition of giant planets.

Aims. We aim to infer the composition and thermal structure of the day-side atmosphere of the ultra-hot Jupiter WASP-121b from two NIRISS/SOSS secondary eclipses observed as part of a full phase curve.

Methods. We extracted the eclipse spectrum of WASP-121b with two independent data reduction pipelines and analysed it using different atmospheric retrieval prescriptions to explore the effects of thermal dissociation, reflected light, and titanium condensation on the inferred atmospheric properties.

Results. We find that the observed day-side spectrum of WASP-121b is best fit by atmosphere models possessing a thermal inversion with temperatures reaching over 3000 K, with spectral contributions from H₂O, CO, VO, and H⁻, along with either titanium oxide (TiO) or reflected light. We found the atmosphere of WASP-121b to be metal-enriched (~10× stellar), but comparatively titanium-poor (≲1× stellar), potentially due to partial cold-trapping. The inferred C/O depends on model assumptions, such as whether reflected light is included, ranging from being consistent with stellar, if a geometric albedo of zero is assumed, to being super-stellar for a freely fitted value of Ag = 0.16_−0.02^+0.02. The volatile-to-refractory ratio was found to be consistent with the stellar value.

Conclusions. From the NIRISS eclipse spectrum, we infer that WASP-121b has an atmosphere that is enriched in both volatile and refractory metals, but not in ultra-refractory titanium, suggesting the presence of a night-side cold-trap. Considering H₂O dissociation is critical in free retrieval analyses, leading to order-of-magnitude differences in retrieved abundances for WASP-121b if neglected. Simple chemical equilibrium retrievals that assume all species are governed by a single metallicity parameter tend to drastically overpredict the TiO abundance, strongly biasing the inferred atmospheric composition.