The First Spin-Orbit Obliquity of an M dwarf / Brown Dwarf System: An Eccentric and Aligned TOI-2119 b

We present the first spin–orbit obliquity measurement for an M dwarf–brown dwarf system, TOI-2119, using the Rossiter–McLaughlin effect. TOI-2119 b is a transiting brown dwarf (64.4 M_J, 1.08 R_J) on an eccentric (e = 0.3), 7.2-day orbit around a young, active early M dwarf (Teff = 3553 K). We combine spectroscopic transit observations from NEID with simultaneous ground-based transit photometry to refine the system parameters and update the orbital ephemeris. Both the classical and Reloaded Rossiter–McLaughlin techniques yield a low projected obliquity, indicating a well-aligned star–companion configuration. This alignment places TOI-2119 b within a growing population of aligned close-in brown dwarfs and provides important constraints on their formation pathways. In particular, the low obliquity disfavours formation scenarios dominated by strong dynamical interactions or high-eccentricity migration, instead supporting origins linked to disc-driven migration or in situ formation within the protoplanetary disc. TOI-2119, therefore, offers a rare observational benchmark for testing brown dwarf formation and migration theories around low-mass stars, bridging the gap between massive exoplanets and stellar companions.