The Fate of the Milky Way–Andromeda System: To Merge or Not?

Abstract It has long been predicted that the Milky Way (MW) will eventually merge with Andromeda (M31), a view reinforced by Hubble Space Telescope measurements indicating a small M31 transverse velocity. However, using updated Gaia-based proper motions (PMs) and including the dynamical influence of the Large Magellanic Cloud and M33, T. Sawala et al. reported an MW–M31 merger probability of ∼50% within 10 Gyr, leaving the fate of the Local Group (LG) uncertain. Adopting their semianalytic framework, we revisit this problem with the latest and most precise Gaia-based PMs for M31 and M33, corrected for systematic offsets in Gaia astrometry. In our fiducial model, the MW–M31 merger probability rises to 90%, with a median merger time of 6 . 5 − 1.5 + 1.3 Gyr, broadly restoring the classical picture. A sensitivity analysis shows that the merger probability depends strongly on the adopted M31 PM through two channels: a direct effect via the radial-tangential balance of the MW–M31 orbit, and a satellite-mediated effect, where the M31 PM fixes the orbital plane and determines how satellite-induced barycentric reflex motions project onto it, either promoting or suppressing a merger. Given this sensitivity, current measurements, while favoring a high merger probability, remain inconclusive, spanning from 64.7% to 100% across the 2 σ PM region. Future PM measurements with uncertainty of ≲2 μ as yr −1 will be required to reach a firm conclusion, i.e., to constrain the probability range within 10% at the 2 σ level.