Milky Way's Ancient Meal: Scientists Discover Remains of 'Loki' Galaxy

Somewhere in the Milky Way's disk, about 7,000 light-years from Earth, twenty stars are telling a story that astronomers have only just begun to read. These stars are ancient—more than 10 billion years old—and they are poor in metals, meaning they formed in an era before the universe had forged many of the heavier elements we take for granted. Their presence where they shouldn't be, clustered near the galactic disk rather than in the halo where such old stars are typically found, has led researchers to a startling conclusion: they are the scattered remains of a dwarf galaxy that the Milky Way consumed roughly 10 billion years ago. The researchers have named this vanished galaxy Loki, after the Norse god whose true intentions are notoriously difficult to discern.

The discovery, published this month in the Monthly Notices of the Royal Astronomical Society, emerged from painstaking detective work. Federico Sestito, a postdoctoral researcher at the University of Hertfordshire, and his colleagues used data from the European Space Agency's Gaia telescope, which has mapped the movements and chemical compositions of two billion stars across the Milky Way since 2014. From that vast catalog, they identified twenty metal-poor stars in an unexpected location—close to the galactic disk—and then used the high-resolution spectrograph at the Canada-France-Hawaii Telescope atop Maunakea to study them in detail. What they found was striking: all twenty stars shared similar chemical signatures, suggesting they had originated from the same source, a single ancient dwarf galaxy now long dissolved into the larger structure of our own.

The Milky Way itself is a cosmic giant, stretching roughly 100,000 light-years across and containing between 100 billion and 400 billion stars. But it was not always so vast. Over the past 12 billion years, it has grown by consuming smaller galaxies—a process astronomers call galactic cannibalism. When a large galaxy's gravity pulls in a smaller neighbor, it absorbs the smaller galaxy's stars and gas, scattering the remnants across its own structure. These remnants are like archaeological artifacts, clues to the Milky Way's feeding history. By finding and studying them, scientists can reconstruct how our galaxy assembled itself piece by piece.

Metal-poor stars are particularly valuable for this work. The earliest stars in the universe were made only of hydrogen and helium. As they burned, they fused these light elements into heavier ones—carbon, oxygen, iron, and the rest of the periodic table. When those stars exploded, they seeded the cosmos with these new elements, enriching the next generation of stars. Stars that formed early, before much enrichment had occurred, remain poor in metals. They are time capsules. Finding them clustered together, with matching chemical fingerprints, suggests they came from the same ancient system. The fact that these twenty stars were discovered near the disk—a region crowded with younger, metal-rich stars and thick with dust—makes the discovery all the more remarkable. Astronomers had largely searched for such ancient remnants in the halo, the diffuse cloud surrounding the disk. The disk was thought too noisy, too full of younger stellar generations, to hide such secrets.

The orbital patterns of the twenty stars add another layer of intrigue. Eleven of them orbit in the same direction as the galactic disk itself, moving prograde. Nine move in the opposite direction, retrograde. This split is unusual and telling. When a small galaxy is absorbed by a much larger one, the smaller galaxy's stars are torn apart by gravitational forces and scattered into different orbital patterns. But the fact that these particular stars show both prograde and retrograde motion while sharing the same chemical origin suggests something specific about when the merger occurred. If the Milky Way had been large and gravitationally dominant at the time, it would have imposed a single dominant orbital direction on the captured stars. The fact that they scatter across both directions implies the Milky Way was still relatively small and weak when it consumed Loki—perhaps only three to four billion years after the Big Bang, when the universe itself was young.

The implications are significant. Astronomers already know of one major merger event in the Milky Way's past: the collision with the Gaia-Sausage-Enceladus galaxy between 8 and 10 billion years ago. That merger was transformative, helping to reshape the Milky Way from a chaotic, turbulent system into the stable, rotating disk we observe today. If the Loki merger is real and occurred even earlier, it may have been equally important—a foundational event in the galaxy's assembly that has been largely hidden from view. Alexander Ji, an astronomer at the University of Chicago, notes that if Loki is confirmed, it would mean scientists have been missing a crucial chapter of the Milky Way's formation story. The evidence is hard to find because the remnants are scattered and faint, buried in the crowded disk. But if the discovery holds up under further scrutiny, it will force a revision of how we understand our galaxy's past.

Still, caution is warranted. Ji points out that many candidate discoveries of ancient mergers turn out to be extensions of already-known systems rather than entirely new events. The authors of the study have included appropriate scientific reservations in their work. Cara Battersby, a physicist at the University of Connecticut, calls the research intriguing and worthy of further investigation, but notes that alternative explanations exist—the stars could potentially come from multiple merger events rather than a single one. Hans-Walter Rix, director of the galaxy and cosmology department at the Max Planck Institute for Astronomy in Germany, praised the study's methodology, particularly the use of detailed chemical abundances as a fingerprint to identify a common origin despite the stars' different orbital directions. The next step is clear: astronomers need to search larger datasets, apply more sophisticated analysis, and look for additional evidence that might confirm or refute the Loki hypothesis. The Milky Way's ancient meal may yet reveal its secrets.