TKS III: A Stellar Obliquity Measurement of TOI-1726 c

TL;DR

This study measures the obliquity of TOI-1726 c, a young exoplanet, finding it aligned with its star, which supports theories of coplanar, dynamically cold planetary system formation.

Contribution

First obliquity measurement of TOI-1726 c, providing insights into the system's formation and alignment, and testing obliquity excitation scenarios in a young planetary system.

Findings

Obliquity of TOI-1726 c is aligned within measurement uncertainties.

System likely has a coplanar, dynamically cold orbital configuration.

Supports theories that multi-planet systems tend to be aligned.

Abstract

We report the measurement of a spectroscopic transit of TOI-1726 c, one of two planets transiting a G-type star with $V$ = 6.9 in the Ursa Major Moving Group ($\sim$400 Myr). With a precise age constraint from cluster membership, TOI-1726 provides a great opportunity to test various obliquity excitation scenarios that operate on different timescales. By modeling the Rossiter-McLaughlin (RM) effect, we derived a sky-projected obliquity of $-1^{+35}_{-32}~^{\circ}$. This result rules out a polar/retrograde orbit; and is consistent with an aligned orbit for planet c. Considering the previously reported, similarly prograde RM measurement of planet b and the transiting nature of both planets, TOI-1726 tentatively conforms to the overall picture that compact multi-transiting planetary systems tend to have coplanar, likely aligned orbits. TOI-1726 is also a great atmospheric target for understanding differential atmospheric loss of sub-Neptune planets (planet b 2.2 $R_\oplus$ and c 2.7 $R_\oplus$ both likely underwent photoevaporation). The coplanar geometry points to a dynamically cold history of the system that simplifies any future modeling of atmospheric escape.