Milky Way galaxy flipped upside down after mega crash, Sun's path may have changed

The Milky Way may have dramatically flipped upside down billions of years ago after a violent collision with another galaxy, according to new research that sheds light on one of the biggest mysteries surrounding our home galaxy.Astronomers from Durham University have found evidence suggesting that the Milky Way's flat spiral disc changed its orientation by more than 90 degrees following a massive head-on collision around 10 to 11 billion years ago. The findings were presented at the Royal Astronomical Society's National Astronomy Meeting in Birmingham.The discovery could explain why the Milky Way's vast stellar halo, a sparse cloud of ancient stars surrounding the galaxy, rotates unusually slowly, something that has puzzled astronomers since observations from the European Space Agency's Gaia mission revealed the phenomenon. Halo 18 is an example of a galaxy that had a head-on collision and had a disc flip. (Photo: RAS) To investigate, researchers used powerful supercomputer simulations to study the evolution of 25 galaxies similar to the Milky Way over billions of years.They found that galaxies with slowly rotating stellar haloes almost always shared two characteristics: they had undergone a major head-on merger with another galaxy and experienced a dramatic "disc flip," where the galaxy's disc tilted by more than 90 degrees."We already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus," said lead researcher Kirill Batrakov. "So, we think that the Milky Way disc likely flipped in the past."The Gaia-Sausage-Enceladus was a dwarf galaxy that slammed into the young Milky Way roughly 10 to 11 billion years ago.The collision, considered the largest merger in the Milky Way's history, reshaped the galaxy and scattered billions of stars into elongated, sausage-shaped orbits that astronomers still observe today.The study suggests the impact may have been so powerful that it reoriented the entire galactic disc. If true, even the Sun could once have travelled along a very different path through the galaxy than it does today. The discovery could explain why the Milky Way's vast stellar halo, a sparse cloud of ancient stars surrounding the galaxy, rotates unusually slowly. (Photo: ESA) "A disc flip also means most of the Milky Way's stars once moved on very different trajectories than they do today, possibly even our own Sun, meaning our 'stable' spot in the galaxy might not have been so stable for the Solar System's whole lifetime," Batrakov said.The research also hints at a deeper connection between the Milky Way's visible stars and its invisible dark matter halo.Scientists found that the slow rotation of the stellar halo closely mirrors the motion of the surrounding dark matter, suggesting both evolved together as the galaxy gradually swallowed smaller satellite galaxies.Because Earth lies within the Milky Way, astronomers can study its structure in greater detail than any other galaxy.The newly proposed disc flip adds an unexpected chapter to the Milky Way's history and offers fresh clues about how galaxies grow, evolve and interact over billions of years.- EndsPublished By: Sibu Kumar TripathiPublished On: Jul 22, 2026 14:00 IST