The True Stellar Obliquity of a Sub-Saturn Planet from the Tierras Observatory and KPF

TL;DR

This study measures the true obliquity of a sub-Saturn planet around a K dwarf, revealing a well-aligned orbit, which informs theories about planetary system architectures and their formation around cool stars.

Contribution

First measurement of the true obliquity of a sub-Saturn planet using combined Rossiter-McLaughlin and stellar rotation data, providing insights into planetary orbit alignment.

Findings

The planet's orbit is well aligned with its host star's rotation axis.

The true obliquity is approximately 15.6 degrees, indicating alignment.

Supports the idea of bimodal orbit distributions for sub-Saturns around cool stars.

Abstract

We measure the true obliquity of TOI-2364, a K dwarf with a sub-Saturn-mass ($M_p = 0.18\,M_J$) transiting planet on the upper edge of the hot Neptune desert. We used new Rossiter-McLaughlin observations gathered with the Keck Planet Finder to measure the sky-projected obliquity $\lambda = 7$$^\circ$$^{+10^\circ}_{-11^\circ}$. Combined with a stellar rotation period of $23.47\pm0.29$ days measured with photometry from the Tierras Observatory, this yields a stellar inclination of $90^{\circ} \pm 13^{\circ}$ and a true obliquity $\psi = 15.6$$^\circ$$^{+7.7^\circ}_{-7.3^\circ}$, indicating that the planet's orbit is well aligned with the rotation axis of its host star. The determination of $\psi$ is important for investigating a potential bimodality in the orbits of short-period sub-Saturns around cool stars, which tend to be either aligned with or perpendicular to their host stars' spin axes.