A new computer simulation has revealed that the Milky Way galaxy's disk may have changed direction by about 90 degrees in the past, in a cosmic event that may have reshaped the movement of the galaxy's stars and may also have affected the sun's orbit around its center.
This result was reached by a team led by Kirill Patrakov from Durham University in Britain, after they tracked the evolution of 25 Milky Way-like galaxies over billions of years using supercomputers.
Our galaxy consists mainly of a disk containing most of the stars, gas, and the central black hole, surrounded by a more diffuse halo containing older stars.
A dwarf galaxy collided with the Milky Way.
The researchers found in the simulation that galaxies with slow-rotating halos were in some cases directly merged with other galaxies, and also experienced a reversal in the direction of their disk.
Astronomers already have strong evidence that the Milky Way had a major collision with a dwarf galaxy known as Gaia-Sausage-Enceladus, about 8 billion to 11 billion years ago.
That dwarf galaxy had a mass more than 10 billion times that of the sun, and when it collided with the Milky Way, our galaxy’s gravity tore it apart, and its stars spread out into highly elongated orbits within the halo, forming sausage-shaped stellar streams; hence its name.
Patrakov said that scientists already know that the Milky Way has experienced this direct collision, and therefore the team believes that the galaxy's disk may have flipped in the past.
Why did the galaxy's disk flip?
The most important question remains without a definitive answer, as simulations have shown that some galaxies whose disks have flipped have not necessarily undergone a galactic merger, suggesting that there may be more than one mechanism that could cause this change.
Patrakov says: "We believe there may be different mechanisms leading to disk flips, but at this stage we do not know which scenario specifically applies to the Milky Way, and we need to investigate the precise mechanisms further."
The inversion also helps explain the slow rotation of the halo. When the disk changes direction, the surrounding halo does not immediately reorient itself; rather, it needs time for its movement to synchronize with the disk's new direction, resulting in a clear difference between their movements.
Has the sun's orbit also changed?
If the Milky Way disk flipped during the lifetime of the solar system, its effects may have reached the Sun itself.
"The inversion of the disk means that most of the galaxy's stars were moving in the past on paths that were completely different from their current paths, and perhaps our sun was too," says Patrakov, meaning that the seemingly stable position of the solar system today may not have been stable throughout its life in the galaxy.
Artist's concept showing remnants of the Gaia Sausage. Yellow arrows again show the positions and motions of the Gaia Sausage stars in the halo of our Milky Way – © ESA
An artist's rendering showing the remains of the Gaia-Sausage Galaxy within the Milky Way's halo, with arrows indicating the movement of its stars (European Space Agency).
Patrakov presented the research findings during the Royal Astronomical Society's National Astronomy Meeting, held at the University of Birmingham between July 20 and 24, 2026.
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