The Milky Way may have flipped dramatically upside down billions of years ago. Astronomers say a violent head-on collision with another galaxy triggered this cosmic shake-up.
Researchers from Durham University uncovered fresh evidence for the event. They presented their findings at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham.
The discovery helps solve a long-standing puzzle. Data from the European Space Agency’s Gaia mission had shown that the Milky Way’s vast stellar halo rotates unusually slowly. Now scientists have a compelling explanation.
To reach their conclusions, the team ran powerful supercomputer simulations. They tracked the evolution of 25 galaxies similar to our own over billions of years.
The results stood out clearly. Galaxies with slowly rotating stellar halos almost always shared two key traits. First, they experienced a major head-on merger. Second, they underwent a dramatic “disc flip,” where the galactic disc tilted more than 90 degrees.
Lead researcher Kirill Batrakov pointed to a specific event in our galaxy’s past. “We already know that the Milky Way had a massive head-on collision with a galaxy known as Gaia-Sausage-Enceladus,” he explained. “So, we think that the Milky Way disc likely flipped in the past.”
This dwarf galaxy slammed into the young Milky Way roughly 10 to 11 billion years ago. The massive impact reshaped our galaxy in dramatic ways. It scattered billions of stars into the elongated, sausage-shaped orbits that astronomers still observe today.
Moreover, the collision proved powerful enough to reorient the entire galactic disc. As a result, even the Sun may have once followed a very different path through the Milky Way.
“A disc flip also means most of the Milky Way’s stars once moved on very different trajectories than they do today,” Batrakov added. “Possibly even our own Sun. This suggests our ‘stable’ spot in the galaxy might not have been so stable for the Solar System’s whole lifetime.”
The study also reveals deeper connections. The slow rotation of the stellar halo closely matches the motion of the surrounding dark matter halo. Both appear to have evolved together as the Milky Way gradually absorbed smaller satellite galaxies.
Because Earth sits inside the Milky Way, scientists can examine its structure in unmatched detail. This new research adds an exciting chapter to our galaxy’s story. It offers valuable clues about how galaxies grow, evolve, and interact across cosmic time.
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