Andromeda repeats the Milky Way's impossible geometry at higher contrast. Around M31, roughly half the known dwarf satellites lie in an extraordinarily thin plane, hundreds of kiloparsecs across with an aspect ratio near 1:10, and line-of-sight velocities show most of these dwarfs sharing a common rotation sense, a vast co-rotating satellite disk seen nearly edge-on from our vantage point (Ibata et al. 2013; Conn et al. 2013).
The configuration compounds every difficulty the Milky Way's plane poses. In ΛCDM, satellites trace a triaxial halo with mild filamentary anisotropy, so a structure this thin, this extended, and this kinematically coherent is a rare outlier in simulations (Pawlowski 2018; Santos-Santos et al. 2023). The M31 plane adds its own twist: it is strongly lopsided, with most plane members concentrated on the side facing the Milky Way, an asymmetry stochastic accretion has no use for. And because M31's plane and the Milky Way's VPOS exist in the same group, the coincidence cost multiplies: two improbable planes in one Local Group, roughly perpendicular neither to each other nor randomly, in a model where each should already be a lottery win.
The standing strengthened as velocity measurements accumulated: the co-rotation has held, and the lopsidedness has resisted explanation. Full proper motions of the M31 dwarfs, arriving through HST, Gaia, and JWST astrometry, will convert the line-of-sight coherence into a complete dynamical test.