This dim cluster of ancient stars was discovered on the extreme periphery of the Milky Way in early test data from the Rubin Observatory, offering a glimpse of the telescope's impressive abilities and shedding new light on our cosmic neighborhood.
Because UFDGs are so small and faint, even ones in the Milky Way's vicinity are difficult to detect. Such dwarf galaxies orbit ours, similar to how Earth orbits the sun. In a 2020 study, scientists estimated that hundreds of these satellite UFDGs may exist. However, only around 60 have been found so far, Aashay Pai, a graduate student in physics at the University of Chicago and a co-author of the new study, told Live Science in an email.
The Rubin Observatory is located atop the Cerro Pachón mountain in Chile. (Image credit: Hernan Stockebrand)However, the data analyzed in the new study wasn't from the LSST; it came from an earlier test run. This test data, collected between April 2025 and January 2026, was called Early Data Preview 2 (EDP2) and covered roughly 7% of the night sky.
With this approach, the researchers detected the previously unseen UFDG. Their findings were published in August in the journal Research Notes of the AAS, which presents non-peer-reviewed work in progress. In the research article, the team states that the newfound UFDG was detected at the eight-sigma significance limit, meaning the chance that this group of stars is a random collection is about 1 in 1.6 quadrillion.
The Rubin Observatory has helped find a previously unknown galaxy orbiting the Milky Way. Called Aquarius IV, it lies in the center of this photo. (Image credit: Cerny et al. / Research Notes of the AAS)The newly discovered satellite galaxy, which the researchers named Aquarius IV, lies in the direction of the constellation Aquarius. Pai described it as "one of the faintest and most compact UFDGs at a large distance." The analyses suggest that the galaxy lies approximately 359,000 light-years from the Milky Way's center. (For comparison, the Milky Way itself is roughly 100,000 light-years long.)
Simon described the discovery of Aquarius IV as robust. However, Pai cautioned that follow-up observations will be required to confirm the object's identity.
Related storiesBesides fleshing out our map of the nearby universe, studying UFDGs can reveal more clues about the mysterious nature of dark matter, which makes up about 85% of all matter in the universe but doesn't interact with light. Additionally, "the stars in ultra-faint dwarfs are also extremely old, dating back to just after the Big Bang, so they give us a unique window to the properties of the first galaxies that formed," Simon explained.
How well do you know our home galaxy? Find out with our Milky Way quiz!
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