You deserve a daily dose of good news.<\/strong> For the latest in science and the night sky, subscribe to EarthSky\u2019s free daily newsletter.<\/p>\n This month, scientists using the preliminary data from the Vera C. Rubin Observatory said they had already discovered Rubin\u2019s first ultra-faint satellite galaxy orbiting our home galaxy, the Milky Way. They\u2019ve dubbed the new galaxy Aquarius IV<\/em>. It\u2019s one of the faintest of the roughly 60 little galaxies currently known to be orbiting (or closely associated with) the Milky Way. <\/p>\n The scientists say this discovery offers an early glimpse of the new observatory\u2019s power to probe the universe.<\/p>\n The Vera C. Rubin Observatory \u2013 on the summit of Cerro Pach\u00f3n in northern Chile \u2013 features a 3.2-gigapixel camera, roughly the size of a small car, making it the largest digital camera ever built for astronomy. This camera\u2019s resolution is so high that displaying a single full-size image would require hundreds of 4K ultra-HD television screens. Using this camera, astronomers began a 10-year Legacy Survey of Space and Time (LSST) in June 2026. <\/p>\n The new satellite galaxy is one of the first of what scientists expect will be many discoveries. Researchers found the galaxy in Rubin\u2019s Early Data Preview 2 dataset. <\/p>\n The researchers, led by William Cerny of Yale University, published their study in arXiv on August 4, 2026. This study has not yet been peer-reviewed.<\/p>\n Aquarius IV lies about 355,000 light-years from Earth, in the Milky Way\u2019s outer halo. After detecting the galaxy in the Rubin data, the team confirmed the discovery in archival images from a second astronomical camera located in Chile, the Dark Energy Camera installed at the Victor M. Blanco 4-meter (13-foot) Telescope at Cerro Tololo.<\/p>\n Astronomers believe Aquarius IV is an ultra-faint dwarf galaxy. The paper said these types of galaxies: <\/p>\n \u2026 are the least luminous, least chemically enriched and most dark matter-dominated galaxies known, making them powerful probes of galaxy formation and dark matter physics.<\/p>\n<\/blockquote>\n Scientists found the first ultra-faint dwarf galaxy orbiting the Milky Way about 20 years ago. Today, there are roughly 60 other discoveries. But astronomers believe there are still hundreds of small galaxies near our Milky Way left to find.<\/p>\n And it\u2019s already looking like Rubin will have a good chance of discovering them. The Early Data Preview 2 doesn\u2019t even come from the start of the LSST survey. It came from a testing phase, before the survey began, between April 2025 and January 2026.<\/p>\n Aquarius IV has an apparent magnitude of 18.3. That is incredibly faint, but \u2013 by stacking images and filtering out \u201cnoise\u201d \u2013 the LSST survey can see even fainter objects. Still, galaxies like Aquarius IV are incredibly dim; they consist of diffuse stars of low luminosity. So they\u2019re challenging to find!<\/p>\n The stars in Aquarius IV are poor in heavy elements, indicating that they are quite ancient, perhaps about 13 billion years old. That would mean they formed early in the universe\u2019s history.<\/p>\n But the researchers caution that their classification of Aquarius IV as an ultra-faint dwarf galaxy is still preliminary. There is a chance it could be an unusually large globular cluster. The researchers said they need to do more observations to get spectroscopic measurements from more of its stars before they can confirm its true nature.<\/p>\n Ultra-faint dwarf galaxies are the least luminous and least chemically enriched (oldest) galaxies known. They are also believed to be the most dark matter-dominated. Dark matter is a mysterious substance believed to make up roughly 85% of all the universe\u2019s matter (or 27% of the total universe). Dark energy, meanwhile, makes up the majority of the universe. <\/p>\n Astronomers study ultra-faint dwarf galaxies in part because they hold clues about how the first galaxies formed and how dark matter shaped the early universe.<\/p>\n Aquarius IV is already an exciting discovery using just the earliest observations from the Rubin Observatory. As the paper says: <\/p>\n Aquarius IV is among the faintest known Milky Way satellites in the distant halo, demonstrating Rubin\u2019s ability to reveal ultra-faint satellites at the limits of existing surveys, even in its initial phases of operation. The sensitivity of Rubin LSST will steadily increase, and systems similar to Aquarius IV are expected to be detectable with more than 85% efficiency using the same search algorithm applied here. Rubin LSST thus stands poised to revolutionize the census of ultra-faint Milky Way satellites.<\/p>\n<\/blockquote>\n Bottom line: The Vera C. Rubin Observatory has only just begun its survey, yet it has already discovered Aquarius IV, an ultra-faint satellite galaxy of the Milky Way.<\/p>\n Source: Discovery of the Distant, Ultra-Faint Milky Way Satellite Aquarius IV with the Vera C. Rubin Observatory Early Data Preview 2<\/p>\n Read more: Rubin Observatory begins 10-year timelapse of the universe<\/p>\n Read more: Rubin Observatory launches real-time alert system<\/p>\n <\/div>\nNew Milky Way satellite galaxy<\/h3>\n
Tiny galaxy hidden in Milky Way\u2019s outskirts<\/h3>\n
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What else do we know about Aquarius IV?<\/h3>\n
![\"Artist\u2019s](\"https:\/\/earthsky.org\/upl\/2026\/08\/Milky-Way-side-view-Chandra.jpg\")
Why ultra-faint galaxies matter<\/h3>\n
Just the beginning for Rubin<\/h3>\n
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