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

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 |lambda| = 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 & Aring;, potentially indicating atmospheric loss due to photoevaporation. However, due to the lack of preingress data, additional observations are needed to confirm this measurement.