SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: Tentative detection of the planet’s magnetic field
Year: 2026
Type: article
Abstract: Context. Exoplanetary magnetic fields govern atmospheric escape and circulation but have eluded detection in close-in planets. In the ultra-hot Jupiter KELT-9b, atmospheric dynamo is predicted to generate atmospheric fields of 400−4000 G. Aims. We searched for the circular polarization in atomic lines that a magnetic field would imprint, through the Zeeman effect, on the transmission spectrum of KELT-9b. Methods. We observed three transits with SPIRou at the CFHT, on 07 September 2025 and 03 and 06 July 2026, and applied least squares deconvolution to 1398 atomic lines to map the transit-averaged Stokes V and null spectra against the orbital semi-amplitude, K p , and systemic velocity, v sys . Results. The 2025 transit shows a Stokes V signature of V / I c = 9.3 × 10 −5 at K p = 219 ± 11 km s −1 , v = −34.0 ± 1.2 km s −1 , close to the expected planetary velocity. Its false-alarm probability is 1.9 × 10 −4 , and thus it is formally a marginal detection. Both 2026 transits are of comparable sensitivity but neither exhibits a signal at that location, setting 3 σ limits of 4.9 and 6.0 × 10 −5 ; the 2025 amplitude would have been recovered at 4.9 σ and 4.0 σ . Conclusions. The 2025 signature is robust to the tests we applied to it but qualifies as a marginal detection at best. Spurious signals of comparable strengths occur elsewhere in our Stokes V and null maps, and we cannot rule out that our marginal detection is excess noise. It is not reproduced ten months later.
Source: Astronomy and Astrophysics
Authors: A. Lavail, F. Debras, D. Roge Royo, A. López Ariste, N. W. Borsato, J.-F. Donati, P. Petit, J. D. Alvarado-Gómez, L. Jouve, C. Moutou, A. Carmona, B. Klein, J. V. Seidel, L. Arnold, E. Artigau, C. Baruteau, E. Chabrol, R. Cloutier, N. J. Cook, N. B. Cowan, X. Delfosse, N. Heidari, E. Martioli, A. Masson, J. P. Wardenier, V. Yariv
Institutions: Centre National de la Recherche Scientifique, Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, Leibniz Institute for Astrophysics Potsdam, University of Oxford, Université Côte d'Azur, Lagrange Laboratory, Observatoire de la Côte d’Azur, Canada–France–Hawaii Telescope, Université de Montréal, Université Paris Cité, Université Paris Sciences et Lettres, Observatoire de Paris, Sorbonne Université, CY Cergy Paris Université, McMaster University, McGill University, Institut de Planétologie et d'Astrophysique de Grenoble, Université Grenoble Alpes, Institut d'Astrophysique de Paris, Laboratório Nacional de Astrofísica, Consejo Superior de Investigaciones Científicas, Centro de Astrobiología, University of Bern
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: Ministerio de Ciencia, Innovación y Universidades, European Commission, Agence Nationale de la Recherche, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Centre National d’Etudes Spatiales, Universität Wien, Conselho Nacional de Desenvolvimento Científico e Tecnológico, Centre National de la Recherche Scientifique, Uppsala Universitet, Canadian Space Agency, National Research Council Canada, Agencia Estatal de Investigación, H2020 European Research Council, NextGenerationEU
Awards: EXOplanet Atmospheres Through Multi-phase Observations, G1105844351, G1272900390, G15659259, G318412399, G3909124325, G4085408077, G4156530268, G4728539003, G491712853, G4993905653, G5195188938, G5635532810, G593054264, G6242881339, G6520718613, G7307374365, G7395345766, G7805626310, Optimising Radial Velocity ExTraction, G7863371703, G8497050386, G863151887, G8667377315, G8783516428, Understanding exoplanet atmospheres through high-resolutionspectroscopy
OpenAlex ID: W7220663442