A tidally detached super Neptune on a strongly misaligned retrograde orbit

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 H α 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.