Vera C. Rubin Observatory detects Aquarius IV, one of the smallest and faintest satellite galaxies of the Milky Way ever seen

In one of the first major discoveries from the Vera C. Rubin Observatory in Chile, scientists have revealed a previously unknown galaxy orbiting the Milky Way. Described in a study that hasn’t been peer-reviewed, the galaxy, named Aquarius IV, is one of the lightest, faintest and farthest satellite galaxies ever seen.

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.

Ultrafaint dwarf galaxies (UFDGs) are among the tiniest and faintest galaxies in the universe. These dim little clusters contain at most a few hundred thousand stars, compared with the Milky Way’s approximately 100 billion, said Joshua Simon, an astronomer at the Observatories of the Carnegie Institution for Science who was not involved in the new research. Instead, “these galaxies are composed almost entirely of dark matter: 99 to 99.9% of their mass is dark, and only 0.1 to 1% is made up of stars,” Simon told Live Science in an email.

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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)

But the Rubin Observatory’s supersharp vision is poised to increase that total. The telescope’s 3,200-pixel, car-sized camera can detect objects up to 100 million times dimmer than those visible to the naked eye as it takes repeat observations of the night sky every few nights. This makes the observatory well equipped to find UFDGs.

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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.

The researchers used computer models on the EDP2 data to identify UFDGs as groups of stars whose brightness and colors suggested they had evolved together. The researchers assumed that these stars were around 13 billion years old, Pai said — nearly as old as the universe itself. This framework allowed them to distinguish UFDGs from stars and other galaxies in front of or behind the observed areas.

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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 researchers also verified the galaxy’s discovery using older images snapped by the 570-megapixel Dark Energy Camera on the NSF Victor M. Blanco Telescope in Chile; the photos showed faint stars that had been overlooked. Although this analysis had a lower significance limit of six sigma — giving it about a 1-in-100-billion chance of being a random fluke — it still supported the tiny galaxy’s existence.

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.)

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Pai estimated Aquarius IV’s mass at about 1,500 times that of the sun. That makes it lightweight relative to the Milky Way, whose mass equals that of 1.5 trillion suns, according to NASA. Furthermore, the absolute magnitude — or intrinsic brightness — of Aquarius IV is -1.9, making it 14 million times dimmer than the Milky Way.

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.

Besides 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.

Pai noted that the Rubin Observatory is expected to find up to 100 such UFDGs during the LSST survey, which officially began in June. The survey will create the most detailed time-lapse video of the universe ever recorded and is expected to reveal millions of changes in the night sky every night, leading to an untold number of discoveries.

Cerny, W., Pai, A., Drlica-Wagner, A., Pace, A. B., Ferguson, P. S., Geha, M., Tan, C. Y., Campana, S., Carlin, J. L., Crnojević, D., Ji, A. P., Limberg, G., Massana, P., Mau, S., Medina, G. E., Mutlu-Pakdil, B., Sakowska, J. D., Shipp, N., and Stringfellow, G. S. (2026). Discovery of the Distant, Ultra-faint Milky Way Satellite Aquarius IV with the Vera C. Rubin Observatory Early Data Preview 2. Research Notes of the AAS, 10(8), 239. https://doi.org/10.3847/2515-5172/ae993b

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