Will Andromeda hit the Milky Way? The odds are now 50-50

The Milky Way and Andromeda have only about a 50-50 chance of merging within the next 10 billion years, according to a 2025 study using Hubble and Gaia data. That overturns the widely repeated claim that a collision in about 4 to 5 billion years is all but certain.

Andromeda, 2.5 million light-years away, is approaching at about 250,000 miles per hour, and astronomers have known it was headed our way since 1912. The open question was its sideways drift.

Why did astronomers think a collision was certain?

Because the sideways drift looked negligible: in 2012 a Hubble team led by Roeland van der Marel measured it and forecast a direct hit within 5 billion years, ending in a single elliptical galaxy.

Will the Milky Way and Andromeda collide?

Perhaps: the odds of a merger within 10 billion years are now roughly a coin toss. Till Sawala’s team at the University of Helsinki ran 100,000 simulations spanning 22 uncertain variables, and added two satellite galaxies that pull in opposite directions. M33, Andromeda’s large companion, tugs the Milky Way slightly towards Andromeda, while the Large Magellanic Cloud pulls the Milky Way off the orbital plane and away, making a crash a bit less likely.

In about half the runs the galaxies swing past each other, then fall back and merge in the far future as friction between their dark-matter halos drains their orbits. In most of the rest they never get close enough to merge. A head-on collision on the old timetable survives in only about 2% of cases.

“It’s somewhat ironic that, despite the addition of more precise Hubble data taken in recent years, we are now less certain about the outcome of a potential collision.” — Till Sawala, lead author

Earth faces nearer problems: the brightening Sun should make it uninhabitable in roughly 1 billion years. Even in a merger, stars are so far apart that they would not collide, though the Sun might be flung into a new region of the combined galaxy.