Letter to the Editor

A tidally detached super Neptune on a strongly misaligned retrograde orbit

¹ Centro di Ateneo di Studi e Attività Spaziali “G. Colombo” – Università degli Studi di Padova, Via Venezia 15, IT-35131 Padova, Italy
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, IT-35122 Padova, Italy
³ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, IT-35122 Padova, Italy
⁴ INAF – Osservatorio Astrofisico di Catania, Via Santa Sofia 78, IT-95123 Catania, Italy
⁵ Dipartimento di Fisica e Astronomia, Università di Padova, Via Marzolo 8, IT-35121 Padova, Italy
⁶ INAF – Osservatorio Astronomico di Roma, Monte Porzio Catone, IT-00043, Roma, Italy
⁷ Fundación Galileo Galilei-INAF, Rambla José Ana Fernandez Pérez 7, ES-38712 Breña Baja, Spain
⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, IT-90134 Palermo, Italy
⁹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, IT-10025 Pino Torinese, Italy
¹⁰ Carnegie Science Observatories, CA 91101 Pasadena, USA
¹¹ Department of Astronomy, University of Wisconsin, 475 N. Charter Str., WI 53706 Madison, USA
¹² Dipartimento di Matematica e Fisica, Università Roma Tre, Via della Vasca Navale 84, IT-00146 Roma, Italy
¹³ Department of Physics and Kavli Institute for Astrophysics and Space Research, MIT, MA 02139 Cambridge, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 April 2026
Accepted: 20 April 2026

Abstract

The obliquity between a planet’s orbital axis and its host star’s spin axis provides crucial insights into planetary formation and migration. Planets with scaled semi-major axes (a/R_★) large enough to be unaffected by tidal alterations (‘tidally detached’) offer a unique opportunity to study the original obliquity in which the system formed. We therefore observed TOI-1710 b (a/R_★ ≈ 36) in transit using HARPS-N + GIANO-B, collecting high-precision radial velocities to measure the Rossiter-McLaughlin effect. Spectral analysis of the Hα and HeI triple lines was also pursued to evaluate atmospheric photoevaporation. Using our knowledge of the star rotation period (21.5 ± 0.2 d), we estimated a true obliquity of ψ = 149⁺¹¹₋₁₀ deg, which indicates a retrograde motion, thus placing TOI-1710 b among the most misaligned systems – and making it the only one known to orbit a cool star in retrograde motion. The strong misalignment favours a high-eccentricity migration (HEM) origin for this low-density super-Neptune planet in the savanna region, challenging previous findings that claimed a minor role of HEM in this period-radius(-density) domain. Moreover, the strong misalignment and lack of a detected close stellar companion suggest a purely planetary post-migration misalignment, likely due to planet-planet scattering followed by planet-planet Kozai-Lidov oscillations and tidal circularisation.