Around the Andromeda galaxy (M31), roughly half of the known dwarf satellites lie in an extremely thin, hundreds-of-kiloparsecs–wide plane with an aspect ratio of order 1:10, and many of these dwarfs appear to share a common sense of rotation, forming a vast, corotating satellite disk (Ibata et al. 2013; Conn et al. 2013). In the standard ΛCDM picture, satellite galaxies should roughly trace a triaxial dark matter halo with only mild anisotropy from filamentary infall, so a long-lived, very thin, co-rotating plane like that seen around M31—together with its strong lopsidedness toward the Milky Way—appears as a rare outlier in simulations and has been highlighted as a small-scale challenge to ΛCDM (Pawlowski 2018; Santos-Santos et al. 2023).