You may have learned that the Andromeda galaxy is moving pretty much directly towards the Milky Way. You may have also learned that the two galaxies will eventually collide and merge into a single elliptical galaxy. If so, you’re certainly not alone. Predictions of a Milky Way-Andromeda merger are widespread in pop-science media and introductory astronomy textbooks, primarily because studies on the matter have historically reported high merger probabilities. A study from June 2025, however, raised important questions about the validity of these results. By focusing on what we still don’t know about the galaxies involved in this potential collision, the authors demonstrated that the probability of a Milky Way-Andromeda collision may be much lower than previously estimated.

To predict how gravity will affect the motion of a set of galaxies, researchers must know the masses of the galaxies, the densities of their halos, their relative positions, how fast they are moving, and in what direction they are moving. However, obtaining these parameters can be challenging, and thus, their values tend to carry relatively large uncertainties. 

For the most massive galaxies in our immediate neighborhood, nearly all the parameters necessary to predict a Milky Way-Andromeda merger carry non-negligible uncertainties. For instance, according to the most accurate data available at the time of the study, the Milky Way should have a mass of 1 ± 0.2 trillion solar masses, where the ± 0.2 indicates a range of possible values between 0.8 and 1.2 trillion. Naturally, within this range, there are possible masses of the Milky Way that would make a merger less likely, and the authors found that the merger probability is lower for smaller possible masses of the Milky Way. They observed a similar relationship between the merger probability and the masses of Andromeda and M33, and an inverse relationship between the merger probability and the mass of the Large Magellanic Cloud (that is, as the mass goes up, the merger probability goes down).

Overall, the authors found that the uncertainties in the masses of all four galaxies, the motions of Andromeda and M33 perpendicular to our line of sight (i.e., their proper motions), and the distances to Andromeda and M33 all significantly impacted the merger probability. When accounting for these uncertainties, they found there is a nearly 50% probability that the Milky Way and Andromeda will not merge in the next 10 billion years. This implies that even with the most accurate data available, it is not currently possible to predict a Milky Way-Andromeda merger. 

Resources

Sawala, T., Delhomelle, J., Deason, A.J. et al. No certainty of a Milky Way–Andromeda collision. Nat Astron 9, 1206–1217 (2025). https://doi.org/10.1038/s41550-025-02563-1