Y. Carteret, V. Bourrier, L. Parc, A. Winter, R. Allart, I. de Castro Leao, F. Bouchy, V. M. Passegger, C. Cadieux, P. Lamontagne, E. Artigau, F. Baron, S. C. C. Barros, B. Benneke, X. Bonfils, M. Bryan, B. L. Canto Martins, R. Cloutier, E. A. S. Cristo, J. Gagné, N. J. Cook, N. B. Cowan, J. R. de Medeiros, X. Delfosse, R. Doyon, X. Dumusque, D. Ehrenreich, J. I. González Hernández, D. Lafreniere, C. Lovis, L. Malo, C. Melo, L. Mignon, C. Mordasini, F. Pepe, R. Rebolo, J. Rowe, N. C. Santos, D. Ségransan, A. Suárez Mascareño, S. Udry, D. Valencia, G. A. Wade, K. Al Moulla, R de Souza Barros de Amorim, L. Dang, E. Deibert, D. O. Fontinele, T. Forveille, Y. G. C. Frensch, R. Lima Gomes, D. Mounzer, S. Pelletier, R Rosener, B Skinner, A. Srivastava, A. K. Stefanov, V. Vaulato, J. P. Wardenier, D. Weisserman

Abstract
The angle between stellar spin axis and planetary orbital plane can provide key insights into the formation and dynamical evolution of planetary systems. In particular, this measurement in multi-planet systems can be used to further discriminate between different competing migration scenarios. We present six transit observations of the sub-Neptune GJ 3090 b obtained with NIRPS and HARPS. GJ 3090 b is the inner planet of a confirmed multi-planet system orbiting an M dwarf (M2). Using high spectral resolution and high temporal cadence spectroscopic observations, we analyzed the Rossiter-McLaughlin (RM) effect induced by GJ 3090 b to determine its orbital obliquity. Through the RM revolutions technique, we find that the planet is on a retrograde orbit with a derived 3D obliquity of ψ = 136⁺²⁴_-18 deg ψ = 136 - 18 + 24 deg . We find no evidence of massive outer planetary or wide stellar binary companions, which disfavors scenarios involving gravitational perturbations from a massive body and instead points toward a primordial misalignment of the protoplanetary disk. Our results establish GJ 3090 b as the first planet on a retrograde orbit discovered around an M dwarf and the first highly misaligned confirmed multi-planet system without a known massive companion. We further propose late secondary disk accretion around GJ 3090, in which the disk is not expected to be aligned with the stellar spin axis, followed by disk-driven migration as the most likely mechanisms to explain the observed architecture. This work also illustrates the capability of the RM revolutions technique when applied to near-infrared data to probe orbital architectures of the smallest planets around M dwarfs that have remained mostly inaccessible.

Keywords
techniques: radial velocities / planets and satellites: formation / planets and satellites: individual: GJ 3090b

Astronomy & Astrophysics
Volume 713, Article Number L15, Number of pages 10
2026 September

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