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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="∣" open="∣"> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>24</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>14</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>14</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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 &lt; 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 H α 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.