The HD 191939 Exoplanet System is Well-Aligned and Flat

We report the sky-projected spin-orbit angle \lambda for HD 191939 b, the innermost planet in a 6 planet system, using Keck/KPF to detect the Rossiter-McLaughlin (RM) effect. Planet b is a sub-Neptune with radius 3.4 \pm 0.8 R_{\oplus} and mass 10.0 \pm 0.7 M_{\oplus} with an RM amplitude <1 ms^{-1}.... Show more