SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: tentative detection of the planet magnetic field
Abstract
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. We search for the circular polarization in atomic lines that a magnetic field would imprint, through the Zeeman effect, on the transmission spectrum of KELT-9b. 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 orbital semi-amplitude Kp and systemic velocity vsys. The 2025 transit shows a Stokes V signature of V/Ic = 9.3 × 10−5 at Kp = 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, formally a marginal detection. Both 2026 transits are of comparable sensitivity but neither exhibits a signal at that location, setting 3σ limits of $4.9$ and 6.0 × 10−5; the 2025 amplitude would have been recovered at 4.9σ and 4.0σ. 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 exclude that our marginal detection is excess noise. It is not reproduced ten months later.
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BibTeX
@misc{lavail2026spirou,
title = {{SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: tentative detection of the planet magnetic field}},
author = {Alexis Lavail and Florian Debras and David Rogé Royo and Arturo López Ariste and Nicholas W. Borsato and Jean-François Donati and Pascal Petit and Julian D. Alvarado Gómez and Laurène Jouve and Claire Moutou and Andres Carmona and Baptiste Klein and Julia V. Seidel and Luc Arnold and Etienne Artigau and Clément Baruteau and Estelle Chabrol and Ryan Cloutier and Neil J. Cook and Nicolas B. Cowan and Xavier Delfosse and Neda Heidari and Eder Martioli and Adrien Masson and Joost P. Wardenier and Vincent Yariv},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.05951},
eprint = {2610.05951},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.05951},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - SPIRou near-infrared spectropolarimetry of the ultra-hot Jupiter KELT-9b: tentative detection of the planet magnetic field
AU - Alexis Lavail
AU - Florian Debras
AU - David Rogé Royo
AU - Arturo López Ariste
AU - Nicholas W. Borsato
AU - Jean-François Donati
AU - Pascal Petit
AU - Julian D. Alvarado Gómez
AU - Laurène Jouve
AU - Claire Moutou
AU - Andres Carmona
AU - Baptiste Klein
AU - Julia V. Seidel
AU - Luc Arnold
AU - Etienne Artigau
AU - Clément Baruteau
AU - Estelle Chabrol
AU - Ryan Cloutier
AU - Neil J. Cook
AU - Nicolas B. Cowan
AU - Xavier Delfosse
AU - Neda Heidari
AU - Eder Martioli
AU - Adrien Masson
AU - Joost P. Wardenier
AU - Vincent Yariv
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2610.05951
UR - https://arxiv.org/abs/2610.05951
AB - 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. We search for the circular polarization in atomic lines that a magnetic field would imprint, through the Zeeman effect, on the transmission spectrum of KELT-9b. 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 orbital semi-amplitude K_p and systemic velocity v_sys. 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, formally a marginal detection. Both 2026 transits are of comparable sensitivity but neither exhibits a signal at that location, setting 3σ limits of $4.9$ and 6.0 × 10^−5; the 2025 amplitude would have been recovered at 4.9σ and 4.0σ. 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 exclude that our marginal detection is excess noise. It is not reproduced ten months later.
ER -