A Galactic Somersault
The word "flip" sounds dramatic, and the reality is just as stunning, though it happened over hundreds of millions of years. The new research, presented in July 2026 at the Royal Astronomical Society's National Astronomy Meeting, doesn't suggest the galaxy
suddenly tumbled through space. Instead, supercomputer simulations show that a massive, head-on collision billions of years ago could have caused the entire plane of the Milky Way's starry disc to slowly but dramatically reorient itself. Imagine a spinning plate being struck at its edge, causing it to wobble and gradually settle into a new, tilted orientation. That’s a simplified version of what scientists believe happened to our galaxy. This helps explain a long-standing mystery: why the sparse cloud of stars surrounding our galaxy, known as the stellar halo, barely seems to rotate at all.
The Prime Suspect: A 'Sausage' Galaxy
Astronomers have a prime suspect for this cosmic crime: a dwarf galaxy known as Gaia-Sausage-Enceladus. The unusual name comes from 2018 data from the European Space Agency's Gaia mission, which showed that stars from this destroyed galaxy had been stretched into long, sausage-like orbits after the collision. This was the Milky Way's last major merger, a defining moment that happened about 10 billion years ago. Though called a "dwarf" galaxy, the intruder was immense, carrying the mass of over 10 billion suns in its stars, gas, and dark matter. Researchers at Durham University ran simulations showing that a head-on collision with a galaxy of this size was the most likely culprit for causing a "disc flip" and creating the slowly rotating stellar halo we see today.
Cosmic Detectives and Computer Clues
Scientists couldn't observe this event directly, as it happened long before our solar system even formed. Instead, they acted as cosmic detectives, using powerful tools to piece together the evidence. The key was the Gaia space telescope, which has been creating an unprecedented 3D map of over a billion stars in our galaxy, tracking their positions and movements. This data revealed the strange, slow rotation of the stellar halo stars compared to the disc stars, which orbit at a brisk 220 kilometers per second. To understand why, the research team used the Auriga suite of supercomputer simulations to model the evolution of Milky Way-like galaxies. The simulations that ended up looking like our present-day galaxy—with a slow-moving halo—almost all had one thing in common: a massive, head-on merger that caused the disc to flip.
A History Written in the Stars
This colossal flip is just one part of our galaxy's dynamic story. Other collisions have left their mark in different ways. An ongoing, gentler merger with the Sagittarius dwarf galaxy is believed to be the cause of a large warp in our galaxy's disc, like the bent edge of a vinyl record. Data suggests this interaction sends ripples through the galaxy that can trigger massive bursts of star formation. In fact, one such burst coincided with the formation of our own Sun about 4.7 billion years ago, suggesting we may owe our existence to these cosmic clashes. Other research from earlier in 2026 even suggests the Milky Way's dark matter halo is oriented almost vertically to the starry disc, another piece of evidence pointing to a past reorientation.
