A giant wave is rippling through the Milky Way, and scientists don’t know why

For about a century, astronomers have known that the Milky Way's stars orbit its core, and Gaia has precisely tracked their speeds and trajectories. Since the 1950s, scientists have also recognized that the galactic disc is not flat but warped. Then in 2020, Gaia uncovered that this warped disc slowly oscillates over time, similar to the motion of a spinning top.

Now, researchers have identified an enormous wave that moves through the Milky Way, influencing stars tens of thousands of light-years from the Sun. The phenomenon is like a rock dropped into a pond, where the resulting ripples spread outward -- only here, the "ripples" are made of stars, stretching across the galaxy's outer regions.

The newly revealed wave is illustrated in the figure above. Thousands of bright stars, shown in red and blue, are overlaid on Gaia's detailed map of the Milky Way.

In the image on the left, we see our galaxy from a top-down perspective. The right panel shows a side view, cutting vertically through the galactic plane. From this angle, the left portion of the galaxy curves upward while the right side bends downward (this is the warp of the disc). The red and blue regions mark the newly discovered wave: red areas indicate stars located above the warped plane, while blue areas show stars lying below it.

Although no spacecraft can venture beyond the galaxy, Gaia's remarkably precise measurements -- covering all three spatial dimensions (3D) and three components of motion (toward and away from us, and across the sky) -- allow scientists to construct these top-down and edge-on views of the Milky Way.

These maps reveal that the wave extends over a vast section of the disc, affecting stars located about 30,000 to 65,000 light-years from the galactic center (the Milky Way itself measures about 100,000 light-years across).

"What makes this even more compelling is our ability, thanks to Gaia, to also measure the motions of stars within the galactic disc," says Eloisa Poggio who is an astronomer at the Istituto Nazionale di Astrofisica (INAF) in Italy, and led the team of scientists that discovered the wave.

"The intriguing part is not only the visual appearance of the wave structure in 3D space, but also its wave-like behavior when we analyze the motions of the stars within it."

In the edge-on view of the Milky Way linked below ("The Milky Way's great wave in motion"), white arrows show how the stars move. The vertical motion of the stars (represented by these arrows) is slightly shifted sideways compared to the pattern of their positions (shown by the red and blue colors).

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

She compares the phenomenon to a stadium crowd performing a wave. If we could freeze that moment in time, some people would be standing upright, others would just have sat down (after the wave passed), and some would be about to stand (as the wave approaches). Galactic timescales are far longer, but the principle is similar.

In this comparison, the people standing upright correspond to the red regions in Gaia's maps, while those about to rise -- moving upward with the greatest vertical speed -- are represented by the longest white arrows pointing up, just ahead of the wave's crest.

Eloisa and her team detected this remarkable motion by carefully studying young giant stars and Cepheid stars, both of which vary in brightness in predictable ways that make them easy for Gaia to observe across large distances.

Because these stars seem to move with the wave, the researchers suspect that gas in the galactic disc may also participate in this large-scale motion. Newly formed stars could retain information from the gas they were born from, preserving a kind of "memory" of the wave.

The cause of the galaxy's vast oscillations is still uncertain. One possibility is that the Milky Way experienced a past encounter or collision with a smaller, dwarf galaxy, but further analysis is needed to confirm this.

This newly found "great wave" might also have some link to a smaller undulating structure known as the Radcliffe Wave, which lies roughly 500 light-years from the Sun and extends about 9,000 light-years across.

"However, the Radcliffe Wave is a much smaller filament, and located in a different portion of the galaxy's disc compared to the wave studied in our work (much closer to the Sun than the great wave). The two waves may or may not be related. That's why we would like to do more research," Eloisa adds.

"The upcoming fourth data release from Gaia will include even better positions and motions for Milky Way stars, including variable stars like Cepheids. This will help scientists to make even better maps, and thereby advance our understanding of these characteristic features in our home galaxy," says Johannes Sahlmann, ESA's Gaia Project Scientist.