SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: Tentative detection of the planet’s magnetic field
14 affiliations
- 1Univ. de Toulouse, CNRS, IRAP
- 2Leibniz Institute for Astrophysics Potsdam
- 3Department of Physics, University of Oxford
- 4Laboratoire Lagrange, Observatoire de la Côte d’Azur, CNRS, Université Côte d’Azur
- 5Canada France Hawai‘i Telescope Corporation (CFHT)
- 6Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal
- 7LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS
- 8Department of Physics & Astronomy, McMaster University
- 9Department of Physics and Trottier Space Institute, McGill University
- 10Univ. Grenoble Alpes, CNRS, IPAG
- 11Institut d’Astrophysique de Paris, CNRS, UMR 7095, Sorbonne Université
- 12Laboratório Nacional de Astrofísica
- 13Centro de Astrobiología (CAB), CSIC-INTA
- 14Weltraumforschung und Planetologie, Physikalisches Institut, University of Bern
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.
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BibTeX
@article{lavail2026spirou,
title = {{SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: Tentative detection of the planet’s magnetic field}},
author = {A. Lavail and F. Debras and D. Roge Royo and A. López Ariste and N. W. Borsato and J.-F. Donati and P. Petit and J. D. Alvarado-Gómez and L. Jouve and C. Moutou and A. Carmona and B. Klein and J. V. Seidel and L. Arnold and E. Artigau and C. Baruteau and E. Chabrol and R. Cloutier and N. J. Cook and N. B. Cowan and X. Delfosse and N. Heidari and E. Martioli and A. Masson and J. P. Wardenier and V. Yariv},
journal = {Astronomy \& Astrophysics},
year = {2026},
month = oct,
doi = {10.1051/0004-6361/202663128},
url = {https://www.aanda.org/10.1051/0004-6361/202663128},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: Tentative detection of the planet’s magnetic field
AU - A. Lavail
AU - F. Debras
AU - D. Roge Royo
AU - A. López Ariste
AU - N. W. Borsato
AU - J.-F. Donati
AU - P. Petit
AU - J. D. Alvarado-Gómez
AU - L. Jouve
AU - C. Moutou
AU - A. Carmona
AU - B. Klein
AU - J. V. Seidel
AU - L. Arnold
AU - E. Artigau
AU - C. Baruteau
AU - E. Chabrol
AU - R. Cloutier
AU - N. J. Cook
AU - N. B. Cowan
AU - X. Delfosse
AU - N. Heidari
AU - E. Martioli
AU - A. Masson
AU - J. P. Wardenier
AU - V. Yariv
PY - 2026
DA - 2026/10/27
JO - Astronomy & Astrophysics
DO - 10.1051/0004-6361/202663128
UR - https://www.aanda.org/10.1051/0004-6361/202663128
AB - 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.
SN - 0004-6361
SN - 1432-0746
ER -