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

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 the Keck/High Resolution Echelle Spectrometer, we observed a partial transit of the planet and detected an associated radial velocity anomaly. By modeling the Rossiter–McLaughlin effect, we derived a sky-projected obliquity of λ=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 ψ < 43° 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 Hα line at 6564.7 Å, potentially indicating atmospheric loss due to photoevaporation. However, due to the lack of preingress data, additional observations are needed to confirm this measurement.