A Colossal Wave Is Rippling Through The Milky Way, Gaia Data Reveals

The motions of stars in the Milky Way have revealed a giant outward ripple, set off by a massive event sometime in our galaxy's wild past.

In a new analysis of data collected by the Gaia space-mapping spacecraft and a database of pulsating stars, astronomers have measured the up-and-down motions of stars in the outer regions of the galactic disk and found patterns consistent with a wave-like corrugation.

What generated this ripple is unknown, but one possible culprit is an encounter with another galaxy – likely the Sagittarius dwarf galaxy currently interacting with the Milky Way – punching through the galactic disk, like a pebble dropping into a pond.

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This finding suggests, once again, that the Milky Way isn't just a static object hanging out in space, but a dynamically active galaxy still ringing from past and continuing processes.

"Taken together, these findings lead us to explore the hypothesis that there is a vertical wave extending over a large portion of the outer disk that is moving away from the galactic center," the researchers write in their paper.

"This wave, detected in young stellar populations, could primarily be part of the gaseous component of the galactic disk, revealed by the kinematics of the young stars which have inherited the bulk motions of the gas from which they were born."

The positions and motions of the stars in the analysis, mapped against the disk of the Milky Way, viewed from above (left) and side-on (right). (ESA/Gaia/DPAC, S. Payne-Wardenaar, E. Poggio et al., 2025)

It's only been in recent years that astronomers have started to piece together the three-dimensional architecture of the Milky Way galaxy.

That's largely thanks to Gaia, a space-based observatory that spent more than a decade mapping the three-dimensional positions of the galaxy's stars from its orbit around the Sun.

It wasn't just the positions, though. Gaia also collected data on how those stars are moving through the galaxy – information that reveals the ghosts of dead galaxies, for example, and ongoing gravitational interactions that are not obvious at a glance.

One other thing the Gaia data has been clarifying is that the Milky Way's disk is not serene and flat, but warped and corrugated at its extremities – possible evidence that something heavy took place in the galaxy's history.

In the new study, a team of astronomers led by Eloisa Poggio of the Italian National Institute for Astrophysics wanted to dig deeper into this strange behavior.

They turned to two types of stars: around 17,000 young giant stars, out to distances up to 23,000 light-years from the Solar System; and around 3,400 Cepheid variable stars, out to distances up to 49,000 light-years from the Solar System.

Since the Milky Way's stellar disk is about 100,000 light-years across, these samples provide good coverage of the disk.

Next, the researchers used data on these stars from DR3, Gaia's most recent data release at the time the research was being conducted, as well as other surveys to work out how these stars are moving in the Milky Way. Specifically, they were looking for vertical velocity, the signature that the disk of the Milky Way is moving up and down.

Here's where it gets really interesting. Both populations of stars showed the same coherent vertical pattern of movement, with alternating peaks and troughs, like ripples in a pond.

Also like ripples in a pond, the amplitude of these ripples increases with distance from the galactic center, reaching higher above and lower below the galactic plane at the outer reaches of the disk.

Compelling evidence for outward propagating ripples in the Milky Way. (ESA/Gaia/DPAC, S. Payne-Wardenaar, E. Poggio et al., 2025)

"This observed behavior is consistent with what we would expect from a wave," Poggio says.

The researchers don't know exactly where the source of the wave lies or what caused it. The Sagittarius dwarf galaxy is one option. Another option is that it could be related to the recently discovered Radcliffe wave, which stretches 9,000 light-years down one of the Milky Way's spiral arms.

"However, the Radcliffe Wave is a much smaller filament, and located in a different portion of the galaxy's disk compared to the wave studied in our work," Poggio says. "The two waves may or may not be related. That's why we would like to do more research."

The next release of Gaia data, DR4, is expected to drop in December 2026. The team hopes to revisit their strange ripple with a much larger dataset to really get to the bottom of what makes our galaxy shimmy in the sky.

The research has been published in Astronomy & Astrophysics.