The Milky Way ripples like a vast cosmic wave. Gaia’s precise measurements reveal a colossal motion sweeping through the galaxy’s disc, an echo of something mysterious in our galaxy’s ancient past.
The Milky Way is anything but static. It rotates and it wobbles, and new observations from the European Space Agency’s Gaia space telescope now reveal another motion, a giant wave moving outward from the galaxy’s centre.
Astronomers have known that stars orbit the galactic centre, and Gaia has mapped their speeds and paths. Since the 1950s, researchers have recognized that the Milky Way’s disc is warped. In 2020, Gaia showed that this disc also wobbles over time, similar to a spinning top.
It is now clear that a vast ripple influences stellar motions across distances of tens of thousands of light-years from the Sun. Like waves spreading from a stone dropped into a pond, this stellar ripple spans a large stretch of the Milky Way’s outer disc.
The unexpected galactic ripple is illustrated in this figure above. Here, the positions of thousands of bright stars are shown in red and blue, overlaid on Gaia’s maps of the Milky Way.
We look at our galaxy from ‘above’. We see across a vertical slice of the galaxy and look at the wave side-on. This perspective reveals that the ‘left’ side of the galaxy curves upward and the ‘right’ side curves downward (this is the warp of the disc). The newly discovered wave is indicated in red and blue: in red areas, the stars lie above, and in blue areas, the stars lie below the warped disc of the galaxy.
