E. Friden, M. R. Zapatero Osorio, V. Bourrier, C. Lovis, R. Allart, S. Cristiani, P. Di Marcantonio, J. Gonzalez Hernandez, C. J. A. P. Martins, E. Palle, F. Pepe, N. C. Santos, M. Steiner

Abstract

GJ 9827 is a compact system hosting three close-in planets in near-resonant orbits that span the radius gap, a deficit of close-in planets separating super-Earths and mini-Neptunes. We analysed transit observations from the ESPRESSO GTO program acquired on each planet and performed a joint Rossiter-McLaughlin (RM) analysis of the three planets using the ANTARESS workflow. We measured the sky-projected spin-orbit angle of each transiting planet (λ_b = 9₋₆₄^+67°, λ_c = −21₋₄₄^+43°, and λ_d = −1 ± 26°) and found them, within uncertainties, to be consistent with an alignment with the stellar spin axis. These results, derived with the RM Revolutions technique, are fully compatible with a classical analysis of the RM anomaly. By combining information on the stellar radius and rotation period, we derived the true 3D spin-orbit angles. Our results indicate that the star is viewed equator-on, with 3D spin-orbit angles of Ψ_b = 54₋₃₈^+26°, Ψ_c = 46₋₃₀^+22°, and Ψ_d = 33₋₂₄^+16°. The 3D spin-orbit angles are broadly consistent with a coplanar system but remain weakly constrained, with posterior distributions peaking at moderate misalignments. An aligned architecture remains compatible at the 2 σ level. These findings support a scenario of smooth disc-driven migration for the three planets, potentially allowing for a moderate primordial misalignment between the protoplanetary disc and the stellar spin axis. Further observations are encouraged to confirm these conclusions.

Keywords
methods: data analysis / techniques: spectroscopic / planets and satellites: dynamical evolution and stability

Astronomy & Astrophysics
Volume 710, Article Number A242, Number of pages 14
2026 June

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