Scientists confirm Milky Way suffered dramatic 90-degree cosmic flip after ancient galaxy crash.

Our Milky Way looks peaceful today, yet scientists say it once performed a wild cosmic cartwheel that turned the whole system upside down. A new study confirms our galaxy suffered a dramatic major disc flip in its distant history. The vast stellar disk shifted orientation by more than 90 degrees at some point. That violent move dragged our solar system along for the ride without anyone noticing.

This transformation likely followed a brutal head-on collision with another drifting galaxy. Around 10 to 11 billion years ago, the Milky Way smashed into a massive dwarf galaxy called Gaia-Sausage-Enceladus, or the Gaia Sausage for short. We already knew this impact knocked billions of stars into looping sausage-shaped paths. Now researchers say it may have also flipped our entire galaxy.

Dr Kirill Batrakov from Durham University leads the investigation. He stated that since we know a massive head-on collision occurred, the disc likely flipped back then too. The revelation emerged while trying to solve one of the Milky Way's greatest puzzles regarding its structure. Most stars live in the flat spiral disk, a region stretching 120,000 light-years wide and just 1,000 light-years thick.

Surrounding this core is the sparsely populated stellar halo. This enormous shell spans roughly 300,000 light-years but extends over a million light-years at its absolute outer limits. It contains stars pulled into the Milky Way from other galaxies through ancient mergers. What makes this halo so unusual is that it rotates incredibly slowly compared to other galaxies.

The European Space Agency's Gaia mission found stars in this outermost region can take up to a billion years to circle the galactic core once. Until now, nobody had any idea why this would be the case. In their paper presented at the Royal Astronomical Society's National Astronomy Meeting in Birmingham, researchers analyzed simulated evolution of 25 Milky Way-like galaxies.

Scientists believe this ancient flip explains why the stellar halo and outermost disc rotate so sluggishly today. The collision between the Milky Way and Gaia-Sausage-Enceladus caused this disaster roughly 10 to 11 billion years ago. Does a galaxy really have to suffer such violence just to grow?

Artists have drawn an impression of stars colliding, marking their simulated paths with yellow arrows to track the Gaia–Enceladus event. Scientists tracked these virtual galaxies over billions of years, watching them change and evolve. They found a clear pattern: systems with the slowest stellar halos shared two traits. Every single one suffered a head-on collision with another galaxy, and every single one endured a major disc flip.

The Milky Way possesses both a glacial stellar halo and evidence of that violent ancient crash. Therefore, it is highly probable our own galaxy also flipped its disc. This revelation suggests the galaxy we know today might have looked nothing like itself several billion years ago. It behaved differently too. As Dr Batrakov explains, '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.' He goes further to note that this implies our so-called stable spot in the galaxy might not have been so stable for the Solar System's whole lifetime.

Because we live inside the Milky Way, we can study its workings better than any other galaxy in the cosmos, making it the perfect laboratory for testing ideas about galaxy evolution. With this extra knowledge about our own history, scientists can start to make more sense of the baffling variety of cosmic structures out in the universe. A separate image shows how a Milky Way–like galaxy that avoided collision would have evolved without ever experiencing such a flip. Dr Batrakov adds: 'Finding that its disc flipped adds a new chapter to that story, one we must account for when placing the Milky Way in a broader context of other galaxies.' He states plainly: 'What excites me the most is that this complex history can be reconstructed just from present–day observations.'

Researchers also found a deep link between the Milky Way's stellar halo and the rotation of the invisible, but critical, dark matter halo. This hidden disc of undetectable matter makes up the majority of the mass in the galaxy and holds the structure together like a gravitational glue. That means understanding the origin of our own slow–moving stellar halo could help shed light on one of science's greatest mysteries.