The Vera C. Rubin Observatory in Chile has discovered two previously unknown companions of the Milky Way, the tiny galaxy Aquarius IV and the star cluster Rubin-GC1, using only a small fraction of the data it is expected to collect over its 10-year survey, astronomers have reported.
The objects were detected in preview observations taken between April 2025 and January 2026, covering about 7% of the night sky. Their discovery, detailed in two new studies, suggests that many more faint satellites of our galaxy remain to be found as the observatory continues its work.
The NSF-DOE Vera C. Rubin Observatory, located on Cerro Pachón in the Chilean Andes, began its 10-year Legacy Survey of Space and Time (LSST) in June. It is designed to repeatedly image the southern sky with an 8.4-meter telescope and the largest digital camera ever built, capturing 3.2-gigapixel images. One of its key goals is to map the Milky Way's halo, the vast, roughly spherical region that contains scattered stars, gas, dark matter, and faint satellite galaxies.
Aquarius IV is a tiny galaxy on the distant outskirts of the Milky Way, about 355,000 light-years from Earth. Rubin-GC1 is a star cluster located on the opposite side of the galaxy, hidden within a region where thick clouds of cosmic dust make observations especially challenging. Aquarius IV was identified at high confidence by automated search algorithms.
"What I find especially exciting is that these discoveries are just the beginning for Rubin," said William Cerny, an astronomy student at Yale University and lead author of the Aquarius IV study, published in Research Notes of the AAS. "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."
Cerny said the new satellites will offer opportunities to test models of galaxy formation, trace the Milky Way's assembly history, and investigate the nature of dark matter. Ultra-faint galaxies like Aquarius IV are dominated by dark matter, making them particularly valuable for studying the invisible substance.
"Finding new Milky Way satellites in Rubin Observatory data has been an exciting future prospect for most of my career," said Marla Geha, a professor at Yale and co-author on both studies. "It is remarkable this is now a reality."
Currently, about 65 satellite galaxies are known around the Milky Way, but some estimates suggest hundreds more remain undetected.
"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."
Analysis
Why This Matters
- The Rubin Observatory is demonstrating its ability to find extremely faint satellites, which could transform our understanding of the Milky Way's immediate environment.
- Studying ultra-faint dwarf galaxies is one of the best ways to probe the properties of dark matter.
- Cataloging these objects will help astronomers piece together how the Milky Way formed and evolved over billions of years.
Background
Astronomers have identified about 65 satellite galaxies orbiting the Milky Way, but theoretical models predict there should be hundreds more. Many are so faint they have eluded detection by previous telescopes. The Vera C. Rubin Observatory was built specifically to discover these elusive objects. Its Legacy Survey of Space and Time (LSST) began in June, and the preview data came from observations taken before the official start. The two new objects were found in just 7% of the sky, suggesting many more will appear as the survey progresses.
What to Watch
- As Rubin collects more data, the number of known Milky Way satellites is expected to rise significantly, possibly reaching hundreds.
- The first major data releases from the LSST will be closely watched by astronomers seeking ultra-faint galaxies.
- The properties of newly discovered satellites may provide evidence for or against current dark matter models.
Checked
12 claims in this story checked against 1 source: 12 supported.
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- SupportedRubin Observatory has discovered two hidden companions of the Milky Way.Latest Science News -- ScienceDaily: “The powerful new Vera C. Rubin Observatory in Chile has already uncovered two previously unknown companions of the Milky Way”
- SupportedHundreds more Milky Way satellites may be waiting to be found.Latest Science News -- ScienceDaily: “Astronomers have identified approximately 65 satellite galaxies around the Milky Way, but some estimates suggest that hundreds more remain undiscovered.”
- SupportedWilliam Cerny said, “What I find especially exciting is that these discoveries are just the beginning for Rubin.”Latest Science News -- ScienceDaily: ““What I find especially exciting is that these discoveries are just the beginning for Rubin,” said William Cerny, an astronomy student in Yale's Graduate School of Arts and Sciences.”
- SupportedCerny said Rubin's full 10-year survey will provide many more Milky Way satellite discoveries.Latest Science News -- ScienceDaily: ““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,” Cerny said.”
- SupportedMarla Geha said finding new Milky Way satellites in Rubin data had been an exciting future prospect for most of her career.Latest Science News -- ScienceDaily: ““Finding new Milky Way satellites in Rubin Observatory data has been an exciting future prospect for most of my career,” said Marla Geha”
- SupportedGeha said, “It is remarkable this is now a reality.”Latest Science News -- ScienceDaily: ““It is remarkable this is now a reality.””
- SupportedThe two discoveries are Aquarius IV, a tiny galaxy, and Rubin-GC1, a star cluster.Latest Science News -- ScienceDaily: “The first, Aquarius IV, is a tiny galaxy located in the distant outskirts of the Milky Way, approximately 355,000 light-years from Earth. The second, Rubin-GC1, is a star cluster”
- SupportedThe discoveries used only a small fraction of the data Rubin is expected to collect over its 10-year survey.Latest Science News -- ScienceDaily: “Both discoveries are remarkable because astronomers made them using only a small amount of the data Rubin is expected to collect over its lifetime.”
- SupportedPreview observations for the studies were gathered between April 2025 and January 2026.Latest Science News -- ScienceDaily: “For the new studies, astronomers examined preview observations gathered between April 2025 and January 2026.”
- SupportedThe images covered about 7% of the night sky.Latest Science News -- ScienceDaily: “These images covered only about 7% of the entire night sky”
- SupportedAquarius IV is about 355,000 light-years from Earth.Latest Science News -- ScienceDaily: “The first, Aquarius IV, is a tiny galaxy located in the distant outskirts of the Milky Way, approximately 355,000 light-years from Earth.”
- SupportedRubin-GC1 is on the opposite side of the Milky Way, in a region where thick cosmic dust makes observations especially challenging.Latest Science News -- ScienceDaily: “The second, Rubin-GC1, is a star cluster on the opposite side of our galaxy, hidden within a region where thick clouds of cosmic dust make observations especially challenging.”