Atmospheric composition and circulation of the ultra-hot Jupiter WASP-121b with joint NIRPS, HARPS and CRIRES+ transit spectroscopy

¹ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal, Montréal, Québec, Canada
³ Centre Vie dans l’Univers, Faculté des sciences de l’Université de Genève, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland
⁴ Light Bridges S.L., Observatorio del Teide, Carretera del Observatorio, s/n Guimar, 38500, Tenerife, Canarias, Spain
⁵ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁷ Division of Astrophysics, Department of Physics, Lund University, Box 118, SE-22100 Lund, Sweden
⁸ Observatoire du Mont-Mégantic, Québec, Canada
⁹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁰ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹¹ Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA
¹² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹³ Department of Physics, University of Toronto, Toronto, ON M5S 3H4, Canada
¹⁴ Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Campus Universitário, Natal, RN 59072-970, Brazil
¹⁵ Department of Physics & Astronomy, McMaster University, 1280 Main St W, Hamilton, ON L8S 4L8, Canada
¹⁶ Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
¹⁷ Department of Earth & Planetary Sciences, McGill University, 3450 rue University, Montréal, QC H3A 0E8, Canada
¹⁸ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada (Madrid), Spain
¹⁹ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
²⁰ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
²¹ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
²² Bishop’s Univeristy, Department of Physics and Astronomy, Johnson-104E, 2600 College Street, Sherbrooke, QC J1M 1Z7, Canada
²³ Department of Physics, Engineering Physics, and Astronomy, Queen’s University, 99 University Avenue, Kingston, ON K7L 3N6, Canada
²⁴ Department of Physics and Space Science, Royal Military College of Canada, 13 General Crerar Cres., Kingston, ON K7P 2M3, Canada
²⁵ School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW 2109, Australia
²⁶ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden
²⁷ European Southern Observatory (ESO), Av. Alonso de Cordova 3107, Casilla 19001, Santiago de Chile, Chile
²⁸ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06000 Nice, France
²⁹ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, Campus UAB, Cerdanyola del Valles 08193, Spain
³⁰ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castellde-fels (Barcelona), Spain

^★ Corresponding author: valentina.vaulato@unige.ch

Received: 4 July 2025
Accepted: 26 August 2025

Abstract

Ultra-hot gas giants such as WASP-121b provide unique laboratories for exploring atmospheric chemistry and dynamics under extreme irradiation conditions. Uncovering their chemical composition and atmospheric circulation is critical for tracing planet formation pathways. Here, we present a comprehensive atmospheric characterisation of WASP-121b using high-resolution transit spectroscopy across the optical to infrared with HARPS, NIRPS, and CRIRES+ spanning nine transit events. These observations are complemented with five TESS photometric sectors, two EulerCam light curves simultaneous to the HARPS and NIRPS transits, and an extensive radial velocity dataset in order to refine WASP-121b's orbital parameters. A cross-correlation analysis detected iron (Fe), carbon monoxide (CO) and vanadium (V) absorption signals with SNR of 5.8, 5.0, and 4.7, respectively. Our retrieval analysis constrains the water (H₂O) abundance to −6.52_−0.68^+0.49 dex, although its absorption signal is effectively muted by the hydride (H⁻) continuum. We constrained the relative abundances of the volatile and refractory elements - which represents a crucial diagnostic of atmospheric chemistry, evolution, and planet formation pathways. The retrieved abundance ratios are broadly consistent with expected values of a solar composition atmosphere in chemical equilibrium, likely indicating minimal disequilibrium chemistry alterations at the probed pressures (∼10⁻⁴−10⁻³ bar). We update the orbital parameters of WASP-121b with its largest radial velocity dataset to date. By comparing orbital velocities derived from both the radial velocity analysis and the atmospheric retrieval, we determined a non-zero velocity offset caused by atmospheric circulation, ΔK_p = −15 ± 3 km s⁻¹ (assuming M_⋆ = 1.38 ± 0.02 M_⊙), consistent with predictions from either drag-free or weak-drag 3D global circulation models, while we caution the non-negligible dependence on the assumed stellar mass. These results place new constraints on the thermal structure, dynamics, and chemical inventory of WASP-121b, highlighting the power of multi-wavelength high-resolution spectroscopy to probe exoplanetary atmospheres.