An Obliquity Measurement of the Hot Neptune TOI-1694b

TL;DR

This study measures the obliquity of the hot Neptune TOI-1694b, revealing a nearly aligned orbit and discussing the influence of outer giant planets on obliquity distributions in such systems.

Contribution

First obliquity measurement of a small planet with an outer giant, providing insights into planetary system evolution and obliquity bifurcation.

Findings

TOI-1694b has a nearly aligned orbit with obliquity ${9 extdegree}^{+22 extdegree}_{-18 extdegree}$.

Evidence for a polar population of hot Neptunes with periods ≤ 6 days.

Outer giant planets may influence the obliquity distribution of hot Neptunes.

Abstract

We present spectral observations of the multiplanet host TOI-1694 during the transit of TOI-1694b, a 26.1 $M_\oplus$ hot Neptune with a 3.77-day orbit. By analyzing radial velocities obtained from the Keck Planet Finder, we modeled the Rossiter-McLaughlin effect and constrained the sky-projected obliquity to ${9\degree}^{+22\degree}_{-18\degree}$, which is strong evidence for a nearly aligned orbit. TOI-1694b is one of fewer than ten small planets accompanied by confirmed outer giant planets for which the obliquity has been measured. We consider the significance of the outer planet TOI-1694c, a Jupiter-mass planet with a 1-year orbit, and its potential role in influencing the orbit of TOI-1694b to its current state. Incorporating our measurement, we discuss the bifurcation in hot Neptune obliquities and present evidence for an independent polar population. The observed polar planets nearly ubiquitously have periods of $\le 6$ days and mass ratios of $10^{-4}$. Early perturbations by outer companions from resonance crossings in the disk-dispersal stage provide the most compelling explanation for this population. Systems which lack the necessary configuration will retain their primordial obliquity, since hot Neptunes lack the angular momentum needed to realign their hosts on relevant timescales.