Loki: A Hidden Dwarf Galaxy in the Milky Way’s Disk

Astronomers led by Dr. Federico Sestito identified twenty very metal-poor (VMP) stars ~7,000 light-years from the Sun within the Mil Way’s disk. Gaia’s map of >1.8 billion stars (2014-2025) gave motions and metallicities; spectroscopy with the Canada-France-Hawaii Telescope confirmed a shared chemical fingerprint and ages >10 billion years, indicating an ancient dwarf-galaxy origin.

CNN

The Mil Way grew by accreting smaller galaxies over ~12 billion years. Metal-poor stars, formed when the Universe contained only hydrogen and helium, serve as fossil records. Earlier surveys targeted the halo; this study shows remnants can survive deep in the disk despite dust and younger, metal-rich stars.

CNN

Eleven stars are prograde and nine retrograde. Simulations show such a mix only if the merger happened while the Mil Way’s gravity was weak, likely within three to four billion years after the Big Bang.

CNN

Dr. Sestito said the “Loki scenario” explains the coexistence of prograde and retrograde stars, a pattern plausible when the galaxy was infant. Dr. Battersby noted VMP stars are clues to Universe’s first stellar generations and that their chemistry and motion reveal early cosmic conditions. Dr. Rix highlighted detailed elemental abundances as a fingerprint for a shared origin despite opposite orbital directions. Dr. Ji cautioned that while mergers reshape growth, evidence for Loki remains tentative and needs verification.

CNN

Dr. Ji expressed skepticism that Loki is an unknown galaxy, noting many reported merger signatures later match known structures. He also suggested the stars could arise from multiple smaller accretion events rather than a single dwarf. The authors acknowledge age uncertainties and call for larger, more comprehensive datasets to test the hypothesis.

CNN

Age estimates rely on chemical proxies, making precise dating difficult. Competing views of the mixed orbital pattern—single versus multiple mergers—remain unresolved, and no further Loki remnants have been found.

CNN

• “VMP stars have been around for billions of years, holding within them clues to the formation of the Universe’s earliest generations of stars,” — Dr. Cara Battersby, associate professor, University of Connecticut

CNN

• “Studying the metal-poor stars’ composition and motion can unlock details about the conditions and dynamics of the early universe, she added.” — Dr. Cara Battersby

CNN

• “If the Loki scenario is correct, then a system merged with our galaxy could deposit its stars into both prograde and in the opposite direction,” — Dr. Federico Sestito, postdoctoral fellow, University of Hertfordshire

CNN

• “There are lots of little mergers happening all the time, but the really big meals can change the growth history of the Milky Way,” — Dr. Alexander Ji, assistant professor, University of Chicago

CNN

Future investigations will use larger datasets to expand the metal-poor disk-star sample, test the Loki hypothesis, and refine the timing of early Mil Way mergers.

CNN