CERRO PACHÓN, Chile, 29 October 2026 — Preview data from the recently completed Vera C. Rubin Observatory in Chile has revealed two previously unknown companions of the Milky Way: a distant, ultra-faint galaxy called Aquarius IV and a dust-shrouded star cluster named Rubin-GC1, researchers reported on 29 October 2026.
Illustration: The Vera C. Rubin Observatory on Cerro Pachón in Chile, under a star-filled sky.
What Happened
Two new studies, working from Rubin's early preview data, identified the two ultra-faint Milky Way satellites. Aquarius IV sits in the Milky Way's distant outskirts, roughly 355,000 light-years from Earth. Rubin-GC1 is a star cluster on the far side of the Milky Way, inside a murky region shrouded by cosmic dust, in the direction of the constellation Ophiuchus.
The Aquarius IV study, led by William Cerny, an astronomy student in Yale's Graduate School of Arts and Sciences, was published in the journal Research Notes of the American Astronomical Society. The Rubin-GC1 study, led by Aashay Pai of the University of Chicago, has been accepted by The Astrophysical Journal Letters and is available as a preprint on arXiv. Cerny is second author of the Rubin-GC1 study.
The discoveries came from data taken before Rubin's official survey began. For the two studies, astronomers analysed preview data collected between April 2025 and January 2026, covering about 7% of the total night sky. On a sky map built from Gaia satellite data, Rubin-GC1 stands out as a collection of numerous resolved stars, while the much more distant Aquarius IV is barely detectable by eye but was identified at high confidence by automated search algorithms, according to the observatory.
"What I find especially exciting is that these discoveries are just the beginning for Rubin," Cerny said. "If the observatory can find systems this faint and difficult with only its preview data, its full 10-year survey will undoubtedly provide us with many more Milky Way satellite discoveries."
Background
Large galaxies are often orbited by satellites — small galaxies or star clusters — but many are extremely faint and difficult to find. There are about 65 known satellite galaxies of the Milky Way, yet researchers speculate that hundreds more Milky Way satellites remain undetected.
The Rubin Observatory, a ground-based astrophysics facility on Cerro Pachón in the Chilean Andes, began its 10-year Legacy Survey of Space and Time (LSST) in June 2026. One of its main missions is to map the Milky Way's halo — the cloudlike sphere of gas, stars and dark matter surrounding the main part of the galaxy — and find previously hidden satellite galaxies, star clusters and other substructures.
Illustration: Aquarius IV is an ultra-faint satellite galaxy, visible only as a sparse scattering of dim stars.
The observatory is equipped with the 8.4-metre Simonyi Survey Telescope, featuring the largest digital camera ever built, with 3.2 gigapixels. It takes detailed images of the Southern Hemisphere sky with the goal of creating an ultra-wide, time-lapse record of the night sky.
The research teams span Yale, the University of Chicago, the University of Virginia, the University of Washington, Dartmouth College, the University of Tampa, NSF NOIRLab, Duke University, the University of Toronto, the Instituto de Astrofísica de Andalucía in Spain, the University of Colorado-Boulder and Lund University in Sweden.
Why It Matters
Beyond the discoveries themselves, the researchers say the findings may provide valuable information about the nature of dark matter, the invisible scaffolding that makes up most of the matter in the universe. Ultra-faint galaxies such as Aquarius IV, which have a fraction of the stars that a larger galaxy contains, are almost entirely composed of dark matter, according to the researchers.
Finding more of these faint systems gives astronomers new opportunities to test models of galaxy formation, trace the assembly history of the Milky Way and investigate dark matter. As Cerny put it, the new satellites "will give us new opportunities to test models of galaxy formation, trace the assembly history of the Milky Way, and investigate the nature of dark matter."
Marla Geha, a professor of astronomy and physics at Yale and co-author on both studies, said completing the census matters for understanding how the faintest galaxies form. "These discoveries are incredibly promising," Geha said. "Continued discoveries will allow us to complete the census of satellite galaxies around the Milky Way, which is a key to understanding how the faintest galaxies form."
The results also act as an early proof of Rubin's power: if preview data covering only about 7% of the sky can already turn up two companions this faint and difficult, the full survey should reveal many more. "Finding new Milky Way satellites in Rubin Observatory data has been an exciting future prospect for most of my career," Geha said. "It is remarkable this is now a reality."
Illustration: Rubin-GC1, a star cluster glowing through dust on the far side of the Milky Way.
What Happens Next
Rubin's 10-year survey is now under way, and astronomers expect a steady stream of new satellite discoveries as it images the southern sky again and again. Each new faint galaxy or cluster adds a data point for models of how the Milky Way assembled and how dark matter shapes its halo.
Follow-up work will also pin down the properties of Aquarius IV and Rubin-GC1 in more detail — their ages, chemical make-up and orbits — and confirm how they fit into the Milky Way's family of satellites. The Rubin-GC1 paper's formal journal publication is still to come, and further studies of the preview data may already hold more hidden companions waiting to be found.
For now, the two discoveries stand as a sign of what is coming: a far more complete map of our galaxy's neighbourhood, built one faint companion at a time.
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FAQ
What are Aquarius IV and Rubin-GC1?
Aquarius IV is an ultra-faint satellite galaxy about 355,000 light-years from Earth, in the Milky Way's distant outskirts. Rubin-GC1 is a star cluster on the far side of the Milky Way, hidden in a dust-shrouded region toward Ophiuchus.
How were they discovered?
Both were found in Vera C. Rubin Observatory preview data collected between April 2025 and January 2026, covering about 7% of the sky, before the observatory's official 10-year survey began.
Why are ultra-faint galaxies important for dark matter?
Ultra-faint galaxies contain very few stars relative to their mass and are almost entirely composed of dark matter, according to the researchers, making them useful laboratories for studying dark matter's nature.
How many Milky Way satellite galaxies are known?
About 65 satellite galaxies of the Milky Way are known, but researchers speculate that hundreds more remain undetected.
What is the Rubin Observatory's Legacy Survey of Space and Time?
It is Rubin's 10-year survey, begun in June 2026, using the 8.4-metre Simonyi Survey Telescope and its 3.2-gigapixel camera to create an ultra-wide, time-lapse record of the southern sky, including a map of the Milky Way's halo.
Sources
- phys.org, "Rubin observatory reveals two hidden companions of the Milky Way," 29 October 2026.
- William Cerny et al., "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 (2026), DOI: 10.3847/2515-5172/ae993b.
- Aashay Pai et al., "Rubin Observatory Reveals a Dust-Shrouded Halo Globular Cluster in Ophiuchus," arXiv (2026), DOI: 10.48550/arxiv.2609.15872.
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