We're always getting powerful reminders we know almost nothing about our universe. For example, dark matter makes up 85% of the universe's mass, yet we've never directly detected a single particle of it. Scientists have been studying space for over 200 years and its basic make-up remains a complete mystery. Add to that, we're constantly surprised by new moons and rogue planets, gravity behaving in surprising ways, and the biggest unanswered question of all: where did all this come from? Simply put, humans have no idea what is going on. We are just naive passengers on a random planet at the mercy of whatever happens next.
And it turns out, that might be the most unsettling surprise yet.
It was first seen last year by Astronomer Eloisa Poggio and her team at Italy's Istituto Nazionale di Astrofisica. They were analyzing data from the European Space Agency's Gaia space telescope (pronounced GUY-uh). Poggio was studying young giant stars, and stars called Cepheid variables that pulse in brightness. But Gaia keeps an eye on much more than that. The spacecraft maps the positions and movements of nearly two billion stars in our Milky Way galaxy. When Poggio's team analyzed the massive data set, they found something bizarre.
A thing -- or more accurately, a disturbance -- was moving through the galaxy that shouldn't be there. It was a kind of ripple in space, like dropping a stone in a cosmic pond. Except Poggio's team could not see what dropped the stone, or why the ripples were still spreading.
But the wave was, indeed, still spreading. And it was so big, it was moving the stars.
The data showed stars oscillating up and down in a pattern that stretched across staggering distances. This wasn't random stellar motion. This was organized, structured, persistent. And when they mapped it in three dimensions, using nearly 17,000 young giant stars and over 3,400 Cepheid variables, the pattern became unmistakable: a great wave was rippling through the Milky Way's outer disc.
The size of the wave is so enormous it's hard to comprehend.
The entire Milky Way galaxy is 100,000 light years across, so far that, by definition, it would take light 100,000 years to travel the distance. The wave spans more than half that distance, affecting stars between 30,000 and 65,000 light-years from the galactic center. And scientists can still see it moving.
When viewed from above, the wave looks like concentric ripples spreading through the spiral arms. But when viewed edge-on, the true nature becomes visible: regions where stars lie above the warped galactic disc and regions where they dip below it, creating an undulating pattern that extends for at least 10,000 light-years--possibly as much as 20,000 light-years.
The amplitude (the height of the wave) reaches over 600 light-years of vertical displacement. Entire solar systems are being shifted across space, hundreds of light years from their 'normal' place in the Galaxy. Millions of stars and planets are taking journeys they didn't sign up for, as the wave moves them up and down the equivalent of 20% of the total thickness of the Milky Way galaxy's disc. It's an interstellar journey forced by a passing wave of unknown origin.
It's yet another phenomenon scientists don't (yet) understand. Not too comforting if the wave is coming your way.
One possible cause? Our galaxy collided with a dwarf galaxy in the ancient past. Seems strange to imagine two groupings of stars and planets 'crashing', but it can actually happen, and the results send shockwaves rippling through the galaxy. Gaia has found evidence it's happened before. This current wave could be an echo of an ancient interstellar crash.
Another theory: the wave might be related to the Radcliffe Wave, another rippling structure discovered much closer to us, only 500 light-years from the Sun. But scientists can't say whether these two waves are connected or completely separate phenomena. And if they are related, no one knows what kind of force could create wave patterns at such vastly different scales. The fact there is another, smaller wave out there doesn't really help explain this massive disturbance in the Milky Way.
In the meantime, the wave is moving outward, away from the galactic center, like ripples spreading from a disturbance. The stars caught in its crest are being carried along, oscillating vertically while simultaneously drifting toward the outer reaches of the galaxy.
If you're wondering when it's going to hit the Earth, the answer is worse than you might imagine. There's a real possibility it already has. We could be riding the wave right now.
A paper published this year titled, "A large-scale vertical corrugation in the Milky Way's young disc" suggests that the "cosmic wave" could be passing through our region of the Galaxy. Because the wave involves very slow, large-scale vertical and radial motions of stars and gas (a few miles per second over thousands of light-years), its local effects are subtle and invisible against normal stellar movement. Scientists would not notice it, because current instruments can only measure how stars move relative to one another. They aren't designed to see if the entire region of space is shifting.
Yet that's exactly what could be happening as you read this. Our entire solar system could be oscillating up and down through the galactic plane, riding a wave we never knew existed until 2024.
Over the lifetime of this wave, our solar system could be displaced vertically by 500 light-years or more. That's enough to take us through completely different galactic neighborhoods. Different radiation environments. Different densities of interstellar gas and dust. Different gravitational influences from nearby star clusters. Different exposure to cosmic rays streaming through space.
You won't feel it. The timescales are too vast, the motion too gradual. You can't sense movement through space at these scales any more than you can feel the Earth rotating beneath your feet or orbiting the Sun at 67,000 miles per hour.
The wave itself will continue for millions of years. If we're possibly riding this cosmic swell through space, we can save a lot of resources building space ships to explore the universe, because we may be exploring it whether we like it or not.
Hopefully Earth won't end up somewhere it was never supposed to be.