Full Breakdown
Milky Way May Have Flipped Its Galactic Disc Billions of Years Ago
7/23/2026, 11:15:06 AM
Core Discovery: Disc-Flip Hypothesis
A team of astronomers led by Kirill Batrakov of Durham University used the Auriga suite of cosmological zoom-in simulations to investigate why the Milky Way’s stellar halo rotates only weakly (? 10–20 km s?¹). The simulations of 25 Milky-Way-like galaxies showed that those with the slowest-rotating haloes share three characteristics: an early formation epoch, a major head-on merger comparable to the Gaia-Sausage-Enceladus event, and a disc-flip in which the galactic disc reoriented by more than 90°. The authors conclude that the Milky Way likely experienced such a flip during its early history, linking the halo’s slow rotation to a past re-orientation of the disc.
Background & Context
The Milky Way consists of a thin, star-rich spiral disc surrounded by a much larger, sparsely populated thick disc (stellar halo). The halo’s stars are older, metal-poor, and follow eccentric orbits, indicating they were accreted from smaller galaxies. Observations from the European Space Agency’s Gaia mission have measured billions of stellar positions and motions, revealing the halo’s weak net rotation and the remnants of a massive past merger with the dwarf galaxy Gaia-Sausage-Enceladus about 10–11 billion years ago.
Simulation Evidence & Key Numbers
- Auriga simulations: 25 Milky-Way analogues evolved from redshift z ? 2.5 (? 11 billion years ago) to the present.
- Common traits of slow-halo galaxies: early formation, a Gaia-Sausage-type merger, and a disc-flip event.
- Halo rotation: simulated galaxies matching the Milky Way show a net halo rotation of roughly 10–20 km s?¹.
- Disc-flip magnitude: re-orientation of the galactic disc by > 90°.
- Dark-matter halo: the same simulations suggest a slowly rotating dark-matter halo that may have co-evolved with the stellar halo.
Official Statements & Responses
They also note that the flip scenario offers a plausible connection between the stellar halo’s slow rotation and the rotation of the invisible dark-matter halo, both of which appear to have evolved together as the galaxy accreted smaller satellites such as the Magellanic Clouds and the Sagittarius Dwarf.
Verbatim Quotes
- “We already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus (often simply called the Gaia Sausage). So, we think that the Milky Way disc likely flipped in the past,” — Kirill Batrakov, explained astronomer
- “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,” — Kirill Batrakov, explained astronomer
Implications for Galactic Evolution
If the Milky Way’s disc indeed flipped, many of its stars—including the Sun—would have migrated onto markedly different orbital paths, challenging the notion of a permanently stable solar environment. The disc-flip model also provides a framework for interpreting the twisted orientation of the Milky Way’s dark-matter halo observed in recent Gaia-based studies, suggesting that stellar and dark-matter components can share a coupled dynamical history.
