MILKY WAY FLIPPED SIDEWAYS IN ANCIENT GALACTIC COLLISION

Astronomers at Durham University have found evidence that the Milky Way was rotated more than 90 degrees following a head-on collision with a dwarf galaxy approximately 10 billion years ago. The incoming galaxy, known as the Gaia Sausage, struck the Milky Way directly, was torn apart, and had its stars absorbed by the larger galaxy. The collision's chaos probably flipped the entire disc of the Milky Way into the position observed today, according to researchers who used supercomputer simulations to demonstrate the effect. Kirill Batrakov, lead researcher on the project, stated that the reorientation probably took at least a few hundred million years to complete.

The discovery emerged from investigation into a longstanding astronomical puzzle. Data from the European Space Agency's Gaia mission showed that stars in the Milky Way's disc orbit the galactic centre at approximately 220 kilometres per second, but stars in the stellar halo—a sparse, roughly spherical cloud of ancient stars surrounding the galaxy—rotate far more slowly at about 25 kilometres per second. The Durham team conducted computer simulations of Milky Way-like galaxies to explain this rotational disparity. Most of the halo's stars originated in smaller galaxies that merged with the Milky Way over time.

The research was presented at the 2026 Royal Astronomical Society's National Astronomy Meeting. Batrakov described the connection between the stellar halo's rotation and disc flips as unexpected, reflecting the complex nature of the Milky Way and galaxies broadly. The findings contribute to growing evidence that the Milky Way has experienced considerable upheaval across its 13.6-billion-year lifespan.