Did the Milky Way Flip 90 Degrees in Its Youth?

The Milky Way may have undergone a massive, slow-motion structural shift, with its stellar disc rotating more than 90 degrees over billions of years. Research presented by Kirill Batrakov of Durham University at the 2026 National Astronomy Meeting suggests this galactic “somersault” was likely triggered by a historic collision with the Gaia-Sausage-Enceladus galaxy, leaving the Milky Way’s halo spinning at a remarkably sluggish 10 to 20 kilometres per second.

Evidence of an Ancient Galactic Collision

Astronomers have long sought to explain why the Milky Way’s stellar halo rotates so slowly compared to the rapid 220-kilometre-per-second pace of its disc. According to research from the Auriga simulation project, this discrepancy points toward a violent history. Roughly 8 billion to 11 billion years ago, the young Milky Way collided with a dwarf galaxy now known as Gaia-Sausage-Enceladus.

While the term “collision” implies a direct impact, Kirill Batrakov notes that the process is more about gravitational influence. As the two galaxies passed through one another, the larger Milky Way stripped stars from the smaller galaxy, depositing them into elongated, chaotic orbits. These stars now form the “Sausage” structure of the halo, preserving the angular momentum of the vanished dwarf galaxy like an astronomical fossil record.

Decoding the Mechanics of a Disc Flip

A “flip” in this context is defined by the angular-momentum vector—an imaginary axis of rotation—shifting by more than 90 degrees. This wasn’t a sudden event. Simulations suggest the transformation took anywhere from 150 million to 2 billion years, as gravity gradually redirected the disc’s material into a new orientation.

In 2017, a study led by Alis Deason combined early data from the Gaia mission with Sloan Digital Sky Survey images to quantify the halo’s prograde rotation at 14 kilometres per second. Building on this, recent work by Ling Zhu and colleagues using Gaia and LAMOST telescope data found that the outer dark-matter halo sits almost vertically to the stellar disc. This suggests that while the inner galaxy tilted, the outer halo may have retained its original orientation, providing a “forensic” trail for researchers to follow.

Future Research and Galactic Archaeology

Confirming this reorientation remains a complex challenge. Future studies aim to map the “scars” of this event by comparing the chemistry and age of halo stars against the orientation of the dark-matter halo. Batrakov acknowledges that while simulations make the Gaia-Sausage-Enceladus merger the most likely culprit, proving the exact causal link to the flip requires further high-resolution mapping of the galaxy’s outskirts.

The findings, shared at the 2026 National Astronomy Meeting, remain subject to peer review. As technology improves, astronomers expect to untangle how much of the Milky Way’s current shape is a result of this ancient, transformative somersault.

Frequently Asked Questions

• Could we have seen the flip if we were alive back then?
  Likely not. The process took hundreds of millions of years, and the Sun would have shifted alongside the majority of the disc, making the change imperceptible to life on Earth.
• What is the Gaia-Sausage-Enceladus galaxy?
  It is the remains of a dwarf galaxy that merged with the Milky Way billions of years ago, identified by the unique, elongated orbits of its stars in our current halo.
• Is the Milky Way still flipping today?
  No. The process is a historical event that occurred billions of years ago; the galaxy is currently in a more stable configuration.

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