The Vera C. Rubin Observatory in Chile has discovered two previously unknown satellite systems of the Milky Way in early observations covering just 7% of the night sky. The findings suggest that hundreds of faint galaxies, star clusters and other structures may still be hidden around our galaxy—and that Rubin could make many of them visible during its planned 10-year survey.
Two hidden companions of the Milky Way
Located on Cerro Pachón in the Chilean Andes, the recently completed NSF-DOE Vera C. Rubin Observatory is designed to examine the changing sky in unprecedented detail. Its preliminary observations have already revealed two extremely faint objects that had escaped detection.
Large galaxies such as the Milky Way are surrounded by smaller systems known as satellite galaxies. These include dwarf galaxies and star clusters, many of which contain very few stars and are consequently difficult to find. Astronomers have identified about 65 satellite galaxies around the Milky Way, but some estimates suggest that hundreds more remain undiscovered.
Two new studies, including one led by astronomers at Yale University, indicate that Rubin could play a major role in finding these missing objects and completing the picture of our galaxy’s surroundings.
Using observations collected before the observatory’s official launch, researchers identified two previously unknown, exceptionally faint satellites.
The first, Aquarius IV, is a small galaxy on the outskirts of the Milky Way, approximately 355,000 light-years from Earth. The second, Rubin GC1, is a star cluster on the opposite side of the galaxy, concealed in a region where thick clouds of cosmic dust make observations particularly challenging.
Both discoveries are notable because they were made with only a small amount of the data Rubin is expected to collect over its lifetime.
“What I find particularly exciting is that these discoveries are just the beginning for Rubin,” said William Cerny, an astronomy student at Yale School of Arts and Sciences.
Cerny is the lead author of the Aquarius IV study, published in AAS Research Notes, and the second author of the Rubin-GC1 study, published in Astrophysical Journal Letters.
“If the observatory can find such a faint and difficult system with only preview data, a full 10-year search will no doubt yield the discovery of many more Milky Way moons,” Cerny said. “These new satellites give us new opportunities to test models of galaxy formation, trace the history of the Milky Way’s assembly, and investigate the nature of dark matter.”
A powerful telescope begins a 10-year survey
In June, Rubin began the ambitious 10-year Legacy Survey of Space and Time (LSST). The project aims to create highly detailed records of the changing night sky while examining celestial objects that previous telescopes could not detect.
One of the survey’s major scientific goals is to map the Milky Way’s “halo”—the vast, nearly spherical region surrounding the galaxy’s main disk. The halo contains scattered stars, gas and dark matter, along with faint satellite galaxies, star clusters and other structures that preserve clues about the Milky Way’s past.
To conduct the survey, Rubin will use the 8.4-meter Simonyi Survey Telescope and the largest digital camera ever built, which can capture images with a resolution of 3.2 gigapixels. By repeatedly photographing broad sections of the Southern Hemisphere sky, the observatory will build a comprehensive time-lapse record of the universe.
The facility is jointly funded by the National Science Foundation and the U.S. Department of Energy’s Office of Science.
Two discoveries in 7% of the sky
For the new research, astronomers analyzed preview observations collected between April 2025 and January 2026. Although the images covered only about 7% of the night sky, they were sufficient to reveal two previously unknown satellites of the Milky Way.
The result offers an encouraging indication of what Rubin may find as it continues to survey larger areas of the sky.
“Finding new Milky Way satellites from Rubin Observatory data has been an exciting prospect for most of my career,” said Mara Geha, a professor of astronomy and physics in the Yale School of Arts and Sciences and a co-author of both studies. “It’s amazing that this has become a reality.”
The research involved an international collaboration. In addition to Yale, participating institutions included 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 Andalusia Institute for Astronomy in Spain, the University of Colorado Boulder and Lund University in Sweden.
Aashy Pai of the University of Chicago was the lead author of the study describing Rubin-GC1.
Faint satellites could illuminate dark matter
Finding more satellites could do more than expand the known population of galaxies surrounding the Milky Way. These faint systems could also help astronomers investigate one of the universe’s greatest mysteries: dark matter.
Dark matter does not emit or reflect light and cannot be observed directly with conventional telescopes. Scientists infer its existence from its gravitational influence on stars, galaxies and other cosmic structures. It accounts for most of the matter in the universe and provides much of the gravitational framework that holds galaxies together.
Ultra-faint galaxies such as Aquarius IV are especially valuable for dark-matter research. They contain only a small fraction of the stars found in larger galaxies, while dark matter makes up most of their total mass. By studying the properties and movements of these systems, researchers can test theories about how dark matter behaves and how it influences galaxy formation.
The discoveries could also help scientists reconstruct the Milky Way’s long history. The positions, motions and properties of small satellite galaxies can reveal how larger galaxies assembled over billions of years, including how they accumulated matter and interacted with smaller star systems.
Rubin’s early findings may provide additional clues about how the smallest and faintest galaxies in the universe form, evolve and persist.
“These findings are very promising,” Geha said. “Continuing discoveries will enable us to complete the survey of satellite galaxies around the Milky Way, which is key to understanding how the faintest galaxies form.”
Source: www.sciencedaily.com
