Ancient Collision May Have Tilted the Milky Way's Orientation by 90 Degrees
The Milky Way, our galactic home, may have experienced a dramatic tilt in its orientation billions of years ago. New research analyzing the motion and distribution of stars suggests the galaxy underwent a roughly 90-degree flip relative to its original orientation—an event astronomers trace back to a major collision with the Gaia-Sausage-Enceladus dwarf galaxy.
This ancient merger, which occurred approximately 10 billion years ago, would have been a violent event capable of fundamentally reshaping the structure of our young galaxy. The gravitational interaction from such a collision can dramatically alter a galaxy's angular momentum, causing its disk to reorient relative to the surrounding halo of dark matter.
Scientists are piecing together this history by studying the motion of stars that were likely incorporated from the dwarf galaxy during the merger. The characteristic "sausage" shape of the debris—revealed by data from the Gaia space observatory—suggests a high-energy, eccentric orbit that would have delivered enough gravitational torque to tip the Milky Way's disk.
Understanding these ancient collisions helps astronomers reconstruct how galaxies like our own assembled over cosmic time and why they appear in the orientations we observe today. The Milky Way's current tilt may therefore be a lasting imprint of one of the most significant events in its formation.