The Milky Way, by every available measurement of how large galaxies actually develop, is a particular kind of cosmic predator. The galaxy reached its current form not through the slow ordinary growth of a single isolated system but through a succession of mergers with smaller neighboring galaxies, each of which was, on the available evidence, absorbed into the larger structure and broken up across billions of years until its component stars became indistinguishable from the ordinary stars of the Milky Way disk.
The mergers are not hypothetical. The mergers have been documented across decades of careful astronomical observation. The most studied of them is the Gaia-Sausage-Enceladus event, a major merger between the Milky Way and a smaller galaxy that occurred between 8 and 10 billion years ago, and that left detectable traces in the orbital patterns and chemical compositions of stars in the Milky Way halo. The Gaia-Sausage-Enceladus event is widely understood, in the contemporary astronomical literature, as having structurally reset the Milky Way from its early turbulent phase to the more stable disk configuration the galaxy has had ever since.
What is, on close examination, less widely registered is that the Gaia-Sausage-Enceladus event was not the only such merger. The Milky Way has, by every available accounting, absorbed many smaller galaxies across its history, and the leftover traces of those absorbed systems are, in principle, still detectable as anomalous populations of stars whose orbital patterns and chemical compositions do not match the dominant populations of the galaxy that absorbed them. The detecting requires considerable patience and considerable instrumentation. The detecting has been the work of a particular branch of astronomical research that has, in the last several years, been producing some of its more interesting findings.
What the 20 stars actually are
The most recent of these findings was published in May 2026 in the Monthly Notices of the Royal Astronomical Society. The research team identified a group of 20 stars in the Milky Way disk whose chemical compositions and orbital patterns do not, on close examination, match either the dominant disk population or the previously identified merger remnants in the galaxy’s halo. The 20 stars are very metal-poor, meaning they contain almost none of the heavy elements that more recent generations of stars have been built from. The stars formed, on the available chemical evidence, within the first two to three billion years after the Big Bang, which makes them among the oldest stellar populations known.
The structural feature that made the 20 stars worth attending to is not just their age. The structural feature is the combination of their age, their chemical composition, and the specific orbital patterns they share. Eleven of the stars are in prograde orbits, moving in the same direction as the galactic disk’s rotation. Nine of the stars are in retrograde orbits, moving in the opposite direction. The mixed orbital configuration is, on close examination, the puzzle the research team was working to solve.
The standard astronomical interpretation, when one finds stars in prograde and retrograde orbits with similar chemical compositions, is that the stars come from two different parent systems with similar histories. The team’s analysis, however, found that the chemical compositions of the 20 stars are nearly identical across both the prograde and retrograde populations. The near-identical compositions suggest, on the available evidence, that the stars share a common origin rather than coming from two separate systems.
What “Loki” actually refers to
The team named the hypothetical parent system Loki, after the Norse god of mischief, in reference to the way the stars had been hiding in plain sight in the Milky Way disk despite carrying chemical signatures that should have made them detectable as foreign objects much earlier. The naming is, on close examination, doing slightly more work than the standard astronomical naming conventions usually involve. The naming reflects both the trickster quality of the stars’ previous invisibility and the chaotic nature of the ancient merger event that is hypothesized to have brought them into the galaxy.
The hypothesis is that Loki was a small dwarf galaxy that merged with the Milky Way at some point during the galaxy’s early formation, roughly 10 billion years ago, when the Milky Way was still in what astronomers call its infant or growing phase. During this period, the Milky Way’s gravitational potential was considerably weaker than it is today. The weaker gravitational potential, on the available simulation evidence, would have allowed a merging dwarf galaxy to deposit its stars into both prograde and retrograde orbits, which is the structurally unusual feature of the Loki sample that the research team was trying to explain.
The team’s lead author, Federico Sestito, has been explicit about the conditional nature of the finding. The 20 stars are, on the available evidence, structurally consistent with being the remnants of a single dwarf galaxy that merged with the Milky Way under the specific conditions of the galaxy’s early formation. The finding does not, by itself, prove that this is what they are. The finding does, however, place the Loki hypothesis on the table as a serious candidate explanation that the wider research community will, in due course, work to confirm or refute through further investigation.
Why detecting “swallowed” galaxies is so hard
It is worth being precise about why findings of this kind are so structurally difficult, because the wider cultural register has not, on the available evidence, developed particularly good intuitions for the underlying problem.
The Milky Way disk, by every available measurement, contains approximately 100 billion to 400 billion stars. The stars are moving in roughly coordinated orbital patterns calibrated to the gravitational structure of the disk. The dominant populations are, on close examination, structurally homogeneous in their chemical composition, having formed from gas that was already enriched with heavy elements produced by previous generations of stars.
Stars from an absorbed dwarf galaxy, by contrast, would have formed in a smaller and chemically less enriched environment, and would carry chemical signatures that distinguish them from the dominant disk population. The signatures are real. The signatures are also small. The signatures require considerable instrumentation to detect, and considerable patience to interpret correctly.
The recent advances in this kind of research have been driven, in significant part, by the European Space Agency’s Gaia mission, which has been producing high-precision measurements of the positions, velocities, and chemical compositions of more than a billion stars in the Milky Way since 2013. The Gaia data, combined with spectroscopic follow-up observations from ground-based telescopes, has made it possible to identify anomalous stellar populations at a level of detail that was simply not available to previous generations of astronomers. The Loki finding is, in some real way, one of the products of this expanded observational capacity.
What the wider implication actually is
The structural implication of findings like Loki, on close examination, is that the Milky Way is considerably more of a composite object than the standard cultural framing of the galaxy has tended to imply. The framing has been, for most of the wider register’s history, that the Milky Way is the home galaxy, a singular and stable cosmic entity within which the sun and the various other stars are embedded. The accurate framing is that the Milky Way is, more accurately, the cumulative product of billions of years of mergers, accretions, and absorptions of smaller systems, the remnants of which are still detectable as anomalous populations within the disk and halo.
The implication is, in some real way, that the galaxy is a historical record rather than a stable structure. The historical record contains, on the available evidence, evidence of the various smaller galaxies that contributed to the Milky Way’s formation, and the patient work of reading this record is, more accurately, what the wider field of galactic archaeology has been calibrated to do.
The Loki finding is one of the more recent contributions to this reading. The reading is incomplete. The reading will, in the coming decades, continue to be filled in by further observations, further analyses, and further identifications of anomalous stellar populations within the Milky Way. The wider picture that emerges from this reading is, on the available evidence, considerably more interesting than the standard cultural framing of the galaxy has been allowing for. The galaxy has been eating its neighbors for billions of years. The galaxy is, in some real way, still digesting them. The work of finding the remains is what the contemporary research is, in some real way, calibrated to do.
The acknowledgment this article wants to leave
The discovery of the 20 unusual very metal-poor stars in the Milky Way disk, published in May 2026 in the Monthly Notices of the Royal Astronomical Society, may turn out to be the identification of the remnants of an ancient dwarf galaxy that the Milky Way absorbed approximately 10 billion years ago. The hypothetical parent system has been named Loki. The naming reflects both the trickster quality of the stars’ previous invisibility within the disk and the chaotic nature of the ancient merger that is hypothesized to have brought them into the galaxy.
The finding is, on close examination, not yet definitive. The 20 stars are structurally consistent with being the remnants of a single absorbed dwarf galaxy. The finding does not, by itself, prove that this is what they are. The wider research community will, in due course, work to confirm or refute the hypothesis through further investigation.
What the finding does, in some real way, regardless of whether the Loki hypothesis is ultimately confirmed, is contribute one more piece to the ongoing reconstruction of the Milky Way’s history as a composite object built up from the absorption of many smaller systems across billions of years. The reconstruction is incomplete. The reconstruction is, on the available evidence, considerably more interesting than the standard cultural framing of the galaxy has been allowing for. The galaxy is not, on close examination, the singular and stable home the wider register tends to treat it as. The galaxy is, more accurately, the cumulative record of billions of years of cosmic appetite, with the leftovers still detectable to anyone willing to do the patient archaeological work of finding them.
