The $\sim$50-Myr-Old TOI-942c is Likely on an Aligned, Coplanar Orbit and Losing Mass

TL;DR

This study observes a young Neptunian exoplanet, TOI-942c, revealing it has an aligned, prograde orbit and shows potential signs of atmospheric loss, providing insights into early planetary evolution.

Contribution

First measurement of the orbital alignment of TOI-942c, a young Neptunian planet, and evidence suggesting atmospheric escape in a young planetary system.

Findings

TOI-942c has a prograde, likely aligned orbit with a true obliquity less than 43 degrees.

Detected blueshifted H-alpha absorption hinting at atmospheric loss via photoevaporation.

The planet is a promising target for studying early atmospheric evolution of Neptunian planets.

Abstract

We report the observation of the transiting planet TOI-942c, a Neptunian planet orbiting a young K-type star approximately 50 Myr years old. Using Keck/HIRES, we observed a partial transit of the planet and detected an associated radial velocity anomaly. By modeling the Rossiter-McLaughlin (RM) effect, we derived a sky-projected obliquity of $\left|\lambda\right|=24^{+14}_{-14}$ degrees, indicating TOI-942c is in a prograde and likely aligned orbit. Upon incorporation of the star's inclination and the planet's orbital inclination, we determined a true obliquity for TOI-942c of $\psi< 43$ degrees at 84\% confidence, while dynamic analysis strongly suggests TOI-942c is aligned with stellar spin and coplanar with the inner planet. Furthermore, TOI-942c is also a suitable target for studying atmospheric loss of young Neptunian planets that are likely still contracting from the heat of formation. We observed a blueshifted excess absorption in the H-alpha line at 6564.7 \AA, potentially indicating atmospheric loss due to photoevaporation. However, due to the lack of pre-ingress data, additional observations are needed to confirm this measurement.