POSEIDON II: The Anti-Aligned Orbit of the Warm Neptune TOI-1710 A b

TL;DR

This study measures the extreme orbital misalignment of the warm Neptune TOI-1710 A b, suggesting a distant companion influences its orbit through dynamical coupling.

Contribution

First measurement of a nearly anti-aligned orbit for a warm Neptune, proposing a dynamical mechanism involving a distant companion.

Findings

The planet has a sky-projected obliquity of 179±19 degrees.

The true obliquity is estimated at 158 degrees, indicating a highly retrograde orbit.

A distant companion likely influences the planet's orbit through dynamical coupling.

Abstract

We present an observation of the Rossiter-McLaughlin effect for the warm-Neptune system TOI-1710 obtained with the NEID spectrograph on the WIYN 3.5 m telescope. These observations reveal that the planet orbits in the opposite direction to the stellar spin, with a sky-projected obliquity $\lambda=179\pm19^{\circ}$. Combined with information about the rotation period of the host star, we measure a true obliquity of $\psi=158_{-13}^{+11}\,^{\circ}$. The host star has an M-dwarf companion at a separation of $\sim3600$ 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 $\sim5\,M_J$ planet on a $\sim15$-au orbit that is nearly aligned with the transiting planet's orbit.