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.

In the left image, we look at our galaxy from ‘above’. On the right, 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.

Even if no spacecraft can travel beyond our galaxy, Gaia’s uniquely accurate vision – in all three spatial directions (3D) plus three velocities (moving towards and away from us, and across the sky) – is enabling scientists to make these top-down and edge-on maps.

From these, we can see that the wave stretches over a huge portion of the galactic disc, affecting stars around at least 30–65 thousand light-years away from the centre of the galaxy (for comparison, the Milky Way is around 100 thousand 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 behaviour when we analyse the motions of the stars within it.”