"The fact that there is only around a 50-50 chance of a merger was very surprising."

The team found only a 2 percent probability that the galaxies will collide in the next five billion years. In slightly over half of the simulated scenarios, Andromeda and the Milky Way experience at least one close encounter before they lose enough orbital energy to collide and merge.

Once seen as inevitable within 10 billion years, the odds are now a coin flip — and the sun will expand to engulf Earth billions of years earlier anyway.