The Milky Way may not have always looked the way we see it today. New research presented at the Royal Astronomical Society’s National Astronomy Meeting suggests that our galaxy’s disc may have undergone a dramatic flip in orientation during its history, rotating by more than 90 degrees.
Astronomers at Durham University used supercomputer simulations of Milky Way-like galaxies to investigate why our galaxy’s stellar halo, the vast, faint cloud of stars surrounding the disc, rotates so slowly. The simulations revealed that galaxies with sluggish haloes often share two features: a major head-on merger with another galaxy and a subsequent disc flip.
Lead researcher Kirill Batrakov explained: “We already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus. So, we think that the Milky Way disc likely flipped in the past.”
That collision, about 10–11 billion years ago, was the largest in the Milky Way’s early history. It reshaped the galaxy and left billions of stars on elongated, sausage-shaped orbits. The new study suggests it may also have tipped the entire disc into a new orientation.
A disc flip means that most stars once moved on very different paths than they do today, including possibly our own Sun. It provides clues about the motion of the Milky Way’s dark matter halo, which appears closely linked to the stellar halo’s rotation. Also, it offers a new way to reconstruct the galaxy’s complex past using present-day observations.
Batrakov noted: “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. What excites me the most is that this complex history can be reconstructed just from present-day observations.”
Not every galaxy experiences a disc flip, but the Milky Way’s history of mergers and reorientation makes it a key testbed for understanding galaxy evolution. By studying its stellar halo and dark matter halo together, astronomers can piece together how galaxies grow, collide, and change shape over billions of years.
It’s a reminder that even our seemingly stable cosmic home has had a turbulent past, and that the Sun’s “quiet orbit” may once have been anything but.
The research is being presented this week at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham.