Some of the oldest stars near Earth may not belong to the Milky Way as astronomers once assumed. Instead, they may be the scattered remains of an ancient dwarf-like system swallowed during the Galaxy’s violent youth, then extended into strange, flattened orbits close to the Galactic plane.
In a recent study published in Monthly Notices of the Royal Astronomical Society, an international team analyzed 20 very metal-poor stars in the nearby stellar region and found that, despite moving in both prograde and retrograde directions, many share unusually similar chemical fingerprints.
The research suggests these stars may have formed in a common environment before being deposited into the Milky Way during its earliest assembly. If so, the researchers tentatively call this lost ancient galaxy “Loki.”
“These findings suggest that the stars formed in an environment that experienced a homogeneous chemical evolution akin to that of dwarf galaxies,” researchers write. “A plausible scenario, supported by cosmological zoom-in simulations, is the early accretion of a single system whose subsequent dynamical evolution naturally produced stars on both prograde and retrograde planar orbits.”
The stars examined in the study are likely extremely old by astronomical standards. They are “very metal-poor,” meaning they contain only tiny amounts of elements heavier than hydrogen and helium.
In astronomy, anything heavier than those two elements is considered a “metal.” Because these stars have so few of them, it suggests they formed very early in the universe, before exploding stars had time to create and spread those heavier elements through space.
This makes metal-poor stars valuable fossils. Their chemical makeups preserve evidence of the first massive stars, early supernova explosions, neutron star mergers, and the small galactic building blocks that eventually merged to form larger galaxies like the Milky Way.
One enduring mystery is where these ancient stars are today. Models of galaxy creation suggest the oldest, most metal-poor stars should be scattered throughout the Milky Way’s halo, a vast, roughly spherical region surrounding the galaxy. However, previous surveys have found that a surprising fraction of very metal-poor stars travel close to the galactic plane, where the Milky Way’s disk resides.
To investigate, researchers selected 20 stars within about 6,500 light-years of the Sun that have planar, highly eccentric orbits. Eleven move in the same general direction as the Milky Way’s rotation, while nine move in the opposite direction.
The stars were observed using the ESPaDOnS high-resolution spectrograph at the Canada-France-Hawaii Telescope, allowing researchers to measure 23 chemical properties, including carbon, magnesium, calcium, titanium, manganese, strontium, barium, and europium.
At first glance, the stars looked broadly similar to ordinary metal-poor halo stars. However, deeper analysis revealed something more unusual. Their chemical abundance patterns were much more tightly clustered than the broader halo population.
In other words, they did not look like a random sampling from the Milky Way’s ancient stellar halo. They looked more like stars that had formed in the same chemically connected environment.
The chemical evidence points to a complex early history. Researchers found signatures consistent with high-energy supernovae and hypernovae, along with contributions from fast-rotating massive stars and neutron star mergers. These events likely produced the elements seen in the stars today, including neutron-capture elements such as strontium, barium, and europium.
Importantly, the stars’ neutron-capture element patterns look more like those found in classical dwarf galaxies than in ultra-faint dwarfs. Ultra-faint dwarf galaxies are small, primitive systems with limited chemical enrichment, while classical dwarf galaxies are larger and better able to retain and mix the material produced by powerful stellar explosions. This suggests the environment where these stars formed was likely more substantial than the smallest galactic building blocks.
Researchers also found no major chemical differences between the prograde and retrograde stars. That is intriguing because stars moving in opposite directions might naturally be expected to come from different events. Instead, their shared chemistry hints that a single disrupted system could have scattered stars into both kinds of orbits.
Cosmological simulations support that possibility. During the Milky Way’s earliest formation, before its disk fully settled, the Galaxy’s gravitational structure was still chaotic.
A dwarf-like system falling into the proto-Milky Way could have deposited stars into a range of orbital configurations, including both prograde and retrograde paths close to the plane. Later mergers, after the disk formed, would have had a harder time producing that same mix.
Researchers modeled what kind of system could have produced the observed stars. If the “Loki” interpretation is correct, the progenitor may have had a baryonic mass comparable to a dwarf galaxy, broadly consistent with the type of system capable of retaining and mixing material from powerful explosions.
Still, the authors are careful not to overstate the case for the existence of a lost “Loki” galaxy. The sample contains only 20 stars, and one appears to be a clear chemical and dynamical outlier.
The broader population of metal-poor stars in planar orbits also appears chemically diverse, suggesting that multiple ancient systems may have contributed to the full population. Loki, if real, may explain only one subset of the mystery.
The study also raises a larger question about how astronomers define the Milky Way’s oldest populations. Some very metal-poor stars near the disk may have formed inside the early Galaxy. Others may have arrived from outside.
At such ancient metallicities, chemical tools that normally distinguish “in situ” stars from accreted stars become less reliable, because the earliest Milky Way and its incoming building blocks may have had overlapping chemical signatures.
Researchers hope that upcoming large spectroscopic surveys such as WEAVE and 4MOST can help solve the mystery. With many more stars analyzed on the same chemical scale, astronomers may be able to determine whether Loki was a real ancient system, one of several buried structures, or part of a more complicated early Milky Way formation story.
For now, researchers say that the findings suggest that hidden among nearby stars are possible remnants of “Loki,” a vanished galactic ancestor, still orbiting close to the Milky Way’s plane billions of years after being torn apart.
“These planar stars are, undoubtedly, showing hints of a distinct star formation site from normal halo stars that are worthy to be taken into account for further follow-up analysis,” researchers conclude. “One intriguing interpretation of the various tests is that these VMPs in planar orbit might be the remnant of an ancient system or, less likely, a pair of systems, sharing a common chemical evolution, that deposited its/their stars during the early Galactic assembly.”
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com