Milky Way flipped after ancient galactic collision, study finds

Milky Way galactic collision Gaia Sausage disk reversal simulations

Astronomers using computer simulations and observations of the Milky Way’s stellar halo conclude that the galaxy likely suffered a frontal collision with a dwarf galaxy, known as Gaia‑Sausage‑Enceladus, around 10‑11 billion years ago. The impact would have caused the galactic disk to rotate dramatically – possibly up to 90 degrees – effectively reversing its orientation.

The result is inferred from the unusually slow rotation of the Milky Way’s halo compared with its disk, a signature seen in simulations of 25 Milky Way‑like galaxies that experienced similar mergers. The research was presented at the National Astronomy Meeting 2026 in Birmingham, organised by the Royal Astronomical Society, and involved UK‑based scientists such as Kirill Batrakov.

The findings imply that not only the Sun’s past trajectory but also the coupled evolution of the stellar halo and the dark‑matter halo were reshaped by this ancient event, offering new insight into the formation histories of large spiral galaxies.

Entities: Durham University · Gaia‑Sausage‑Enceladus · Kirill Batrakov · Milky Way Galaxy · Royal Astronomical Society · University of Durham

Claims

What the coverage asserts, and how well corroborated each claim is across sources.

[● 2 SOURCES] The collision likely altered the Sun’s past orbital trajectory within the galaxy. (Study authors)
[● 2 SOURCES] The research was presented at the National Astronomy Meeting 2026 in Birmingham, organised by the Royal Astronomical Society. (Conference program)
[● 2 SOURCES] Kirill Batrakov led the study that reported the Milky Way’s possible disc reversal. (Study lead author)
[● 2 SOURCES] The Milky Way’s stellar halo rotates markedly slower than its disc, indicating a past merger event. (Observational analysis)
[● 4 SOURCES] The collision may have caused the Milky Way’s disc to rotate up to 90 degrees, effectively reversing its orientation. (Study authors)
[● 2 SOURCES] The evolution of the stellar halo appears linked to the evolution of the Milky Way’s dark‑matter halo. (Authors)
[● 8 SOURCES] The Milky Way collided frontally with the dwarf galaxy Gaia‑Sausage‑Enceladus about 10‑11 billion years ago. (Astronomers)
[● 2 SOURCES] Simulations of 25 Milky Way‑like galaxies show that a slowly rotating stellar halo correlates with a past major merger. (Simulation team)