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

S. Pelletier; L.-P. Coulombe; J. Splinter; B. Benneke; R. J. MacDonald; D. Lafrenière; N. B. Cowan; R. Allart; E. Rauscher; R. C. Frazier; M. R. Meyer; L. Albert; L. Dang; R. Doyon; D. Ehrenreich; L. Flagg; D. Johnstone; A. B. Langeveld; O. Lim; C. Piaulet-Ghorayeb; M. Radica; J. Rowe; J. Taylor; J. D. Turner

doi:10.1051/0004-6361/202556985

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

S. Pelletier^*, L.-P. Coulombe, J. Splinter, B. Benneke, R. J. MacDonald, D. Lafrenière, N. B. Cowan, R. Allart, E. Rauscher, R. C. Frazier, M. R. Meyer, L. Albert, L. Dang, R. Doyon, D. Ehrenreich, L. Flagg, D. Johnstone, A. B. Langeveld, O. Lim, C. Piaulet-GhorayebM. Radica, J. Rowe, J. Taylor, J. D. Turner

^*Corresponding author for this work

Research output: Contribution to journal › Article › peer-review

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.

Original language	English
Article number	A2
Number of pages	19
Journal	Astronomy & Astrophysics
Volume	706
Early online date	28 Jan 2026
DOIs	https://doi.org/10.1051/0004-6361/202556985
Publication status	Published - 1 Feb 2026

Keywords

Techniques: spectroscopic
Planets and satellites: atmospheres
Planets and satellites: composition
Planets and satellites: formation
Planets and satellites: gaseous planets
Planets and satellites: individual: WASP-121b

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© The Authors 2026. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Pelletier, S., Coulombe, L.-P., Splinter, J., Benneke, B., MacDonald, R. J., Lafrenière, D., Cowan, N. B., Allart, R., Rauscher, E., Frazier, R. C., Meyer, M. R., Albert, L., Dang, L., Doyon, R., Ehrenreich, D., Flagg, L., Johnstone, D., Langeveld, A. B., Lim, O., ... Turner, J. D. (2026). Enriched volatiles and refractories but deficient titanium on the day-side atmosphere of WASP-121b revealed by JWST/NIRISS. Astronomy & Astrophysics, 706, Article A2. https://doi.org/10.1051/0004-6361/202556985

@article{62d665c8483c4050abb74a1d685a7e1f,

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

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 (\textasciitilde{}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.",

keywords = "Techniques: spectroscopic, Planets and satellites: atmospheres, Planets and satellites: composition, Planets and satellites: formation, Planets and satellites: gaseous planets, Planets and satellites: individual: WASP-121b",

author = "S. Pelletier and L.-P. Coulombe and J. Splinter and B. Benneke and MacDonald, \{R. J.\} and D. Lafreni{\`e}re and Cowan, \{N. B.\} and R. Allart and E. Rauscher and Frazier, \{R. C.\} and Meyer, \{M. R.\} and L. Albert and L. Dang and R. Doyon and D. Ehrenreich and L. Flagg and D. Johnstone and Langeveld, \{A. B.\} and O. Lim and C. Piaulet-Ghorayeb and M. Radica and J. Rowe and J. Taylor and Turner, \{J. D.\}",

note = "Funding: This project has been carried out within the framework of the National Centre of Competence in Research PlanetS supported by the Swiss National Science Foundation (SNSF) under grant 51NF40\_205606. The authors acknowledge the financial support of the SNSF. R.J.M. acknowledges support by NASA through the NASA Hubble Fellowship grant HST-HF2-51513.001, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. R.A. acknowledges the SNSF support under the Post-Doc Mobility grant P500PT\_222212 and the support of the Institut Trottier de Recherche sur les Exoplane{\`e}tes (IREx). L.D. acknowledges support from the Natural Sciences and Engineering Research Council (NSERC) and the Trottier Family Foundation. D.J. is supported by NRC Canada and by an NSERC Discovery Grant. C.P.-G. acknowledges support from the E. Margaret Burbidge Prize Postdoctoral Fellowship from the Brinson Foundation. J.D.T. acknowledges funding support by the TESS Guest Investigator Program G06165.",

year = "2026",

month = feb,

day = "1",

doi = "10.1051/0004-6361/202556985",

language = "English",

volume = "706",

journal = "Astronomy \& Astrophysics",

issn = "0004-6361",

publisher = "EDP Sciences",

}

Pelletier, S, Coulombe, L-P, Splinter, J, Benneke, B, MacDonald, RJ, Lafrenière, D, Cowan, NB, Allart, R, Rauscher, E, Frazier, RC, Meyer, MR, Albert, L, Dang, L, Doyon, R, Ehrenreich, D, Flagg, L, Johnstone, D, Langeveld, AB, Lim, O, Piaulet-Ghorayeb, C, Radica, M, Rowe, J, Taylor, J & Turner, JD 2026, 'Enriched volatiles and refractories but deficient titanium on the day-side atmosphere of WASP-121b revealed by JWST/NIRISS', Astronomy & Astrophysics, vol. 706, A2. https://doi.org/10.1051/0004-6361/202556985

TY - JOUR

T1 - Enriched volatiles and refractories but deficient titanium on the day-side atmosphere of WASP-121b revealed by JWST/NIRISS

AU - Pelletier, S.

AU - Coulombe, L.-P.

AU - Splinter, J.

AU - Benneke, B.

AU - MacDonald, R. J.

AU - Lafrenière, D.

AU - Cowan, N. B.

AU - Allart, R.

AU - Rauscher, E.

AU - Frazier, R. C.

AU - Meyer, M. R.

AU - Albert, L.

AU - Dang, L.

AU - Doyon, R.

AU - Ehrenreich, D.

AU - Flagg, L.

AU - Johnstone, D.

AU - Langeveld, A. B.

AU - Lim, O.

AU - Piaulet-Ghorayeb, C.

AU - Radica, M.

AU - Rowe, J.

AU - Taylor, J.

AU - Turner, J. D.

N1 - Funding: This project has been carried out within the framework of the National Centre of Competence in Research PlanetS supported by the Swiss National Science Foundation (SNSF) under grant 51NF40_205606. The authors acknowledge the financial support of the SNSF. R.J.M. acknowledges support by NASA through the NASA Hubble Fellowship grant HST-HF2-51513.001, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. R.A. acknowledges the SNSF support under the Post-Doc Mobility grant P500PT_222212 and the support of the Institut Trottier de Recherche sur les Exoplaneètes (IREx). L.D. acknowledges support from the Natural Sciences and Engineering Research Council (NSERC) and the Trottier Family Foundation. D.J. is supported by NRC Canada and by an NSERC Discovery Grant. C.P.-G. acknowledges support from the E. Margaret Burbidge Prize Postdoctoral Fellowship from the Brinson Foundation. J.D.T. acknowledges funding support by the TESS Guest Investigator Program G06165.

PY - 2026/2/1

Y1 - 2026/2/1

N2 - 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.

AB - 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.

KW - Techniques: spectroscopic

KW - Planets and satellites: atmospheres

KW - Planets and satellites: composition

KW - Planets and satellites: formation

KW - Planets and satellites: gaseous planets

KW - Planets and satellites: individual: WASP-121b

U2 - 10.1051/0004-6361/202556985

DO - 10.1051/0004-6361/202556985

M3 - Article

SN - 0004-6361

VL - 706

JO - Astronomy & Astrophysics

JF - Astronomy & Astrophysics

M1 - A2

ER -

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

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Keywords

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