POSEIDON II. The Antialigned Orbit of the Warm Neptune TOI-1710 A b

We present an observation of the Rossiter–McLaughlin effect for the TOI-1710 system with the NEID spectrograph on the WIYN 3.5 m telescope. The system hosts a warm Neptune (P ∼ 24 days), and our observations reveal that it orbits in the direction opposite to the stellar spin, with a sky-projected obliquity λ = 179° ± 19°. Combined with information about the rotation period of the host star, we measure a true obliquity (Formula presented) ψ = 158⁺¹¹₋₁₃°. The host star has an M dwarf companion at a separation of ∼3600 au, but this companion is too distant to be solely responsible for misaligning the warm Neptune. The host star also shows a long-term radial velocity trend, indicative of a companion at intermediate separations. We show that such a companion can dynamically couple the warm Neptune to the distant M dwarf, enabling the transfer of inclination from the wide binary orbit to the planetary orbit. Assuming this scenario is correct, we predict the intermediate companion is a ∼5 M_J planet on a ∼15 au orbit that is nearly aligned with the transiting planet’s orbit.