A Galaxy Turned on Its Side
Imagine our sprawling, flat galaxy of stars, gas, and dust slowly, but unstoppably, tipping over. That's the core idea behind a new theory from astronomers at Durham University. Using powerful supercomputer simulations, they found compelling evidence
that the Milky Way’s disc likely flipped by more than 90 degrees. This wasn't an instantaneous event, but a gradual reorientation that would have taken hundreds of millions of years to complete. This cosmic upheaval happened around 10 to 11 billion years ago, long before the Sun and Earth were formed.
The Clue in the Stellar Halo
The evidence for this dramatic flip comes from a long-standing cosmic puzzle: the strange behaviour of the Milky Way's stellar halo. The main disc, where our Sun resides, spins rapidly at about 220 kilometres per second. But surrounding this disc is a vast, sparse sphere of ancient stars called the stellar halo. Data from the European Space Agency's Gaia mission revealed that this halo rotates incredibly slowly, at a mere 25 kilometres per second. Scientists wondered why there was such a huge difference. The new simulations provided a stunning answer: galaxies with extremely slow-rotating haloes almost always had two things in common in their past—a major head-on collision and a subsequent disc flip.
The Ghost of a Lost Galaxy
The prime suspect for causing this galactic turnover is a dwarf galaxy known as Gaia-Sausage-Enceladus. This name, given by astronomers, describes the remains of a galaxy that smashed head-on into the young Milky Way billions of years ago. The collision, discovered in 2018 also using Gaia data, was the last major merger in our galaxy's history. It tore the smaller galaxy to shreds, but not before its immense gravitational force delivered a shock that destabilised our entire galactic disc. The stars from this consumed galaxy now populate our halo, moving in distinctive, elongated, sausage-shaped orbits that serve as the smoking gun for this ancient crash.
Our Sun's Ancient Inheritance
While this flip happened long before our solar system existed, it fundamentally reshaped the galaxy that our Sun would eventually be born into. The collision and subsequent reorientation are believed to have helped form the central bulge of the Milky Way and dominate the stellar halo we see today. According to the researchers, a disc flip means that most of the stars in the galaxy, including potentially our own Sun, once moved on very different paths than they do today. Our 'stable' spot in the cosmos might have a far more turbulent and unstable history than we ever imagined, adding a dramatic new chapter to the story of our galactic home.
