POSEIDON II: The Anti-Aligned 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 ($P\sim 24$ days), and our observations reveal that it orbits in the opposite direction to the stellar spin, with a sky-projected obliquity $\lambda =179\pm {19}^{\circ }$. Combined with information about the rotation period of the host star, we measure a true obliquity $\psi =158_{-13}^{+11}{}^{\circ }$. The host star has an M-dwarf companion at a separation of $\sim 3600$ 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 $\sim 5M_J$ planet on a $\sim 15$ au orbit that is nearly aligned with the transiting planet’s orbit.