Rubin Observatory discovers two hidden companions of the Milky Way. Hundreds more may be waiting

The powerful new Vera C. Rubin Observatory in Chile has already uncovered two previously unknown companions of the Milky Way, offering an early glimpse of what could become a much larger wave of discoveries. Astronomers believe hundreds of faint galaxies may still be hiding around our galaxy, and the observatory could finally bring many of them into view.

Located on Cerro Pachón in the Chilean Andes, the recently completed NSF-DOE Vera C. Rubin Observatory is designed to explore the sky in extraordinary detail. Its initial observations have now revealed two elusive objects that had escaped detection, demonstrating the telescope's ability to find some of the faintest members of our galactic neighborhood.

Large galaxies such as the Milky Way are surrounded by smaller companions known as satellites. These include dwarf galaxies and star clusters, many of which contain so few stars that they are extremely difficult to detect. Astronomers have identified approximately 65 satellite galaxies around the Milky Way, but some estimates suggest that hundreds more remain undiscovered.

Two new studies, including one led by a Yale astronomer, suggest that Rubin could play a major role in finding these missing objects and completing the picture of our galaxy's surroundings.

Two Hidden Companions of the Milky Way Discovered

Using observations collected before the observatory's official launch, researchers identified two exceptionally faint satellites that had never been seen before.

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 on the opposite side of our galaxy, hidden within a region where thick clouds of cosmic dust make observations especially challenging.

Both discoveries are remarkable because astronomers made them using only a small amount of the data Rubin is expected to collect over its lifetime.

"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.

Cerny is the lead author of the Aquarius IV study, published in Research Notes of the AAS, and the second author of the Rubin-GC1 study, which has been accepted by The Astrophysical Journal Letters.

"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. "These 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."

A Powerful New Telescope Is Just Getting Started

Rubin began its ambitious 10-year Legacy Survey of Space and Time (LSST) in June. The project aims to produce an exceptionally detailed record of the changing night sky while searching for objects that previous telescopes have been unable to detect.

One of its major scientific goals is to map the Milky Way's "halo," a vast, roughly spherical region that surrounds the galaxy's main disk. This 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 carry out this enormous survey, Rubin uses the 8.4-meter Simonyi Survey Telescope and the largest digital camera ever constructed, capable of capturing images with 3.2 gigapixels of resolution. By repeatedly photographing large areas of the Southern Hemisphere sky, the observatory will build an extraordinarily wide-ranging 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.

Just 7% of the Night Sky Revealed Two New Satellites

For the new studies, astronomers examined preview observations gathered between April 2025 and January 2026. These images covered only about 7% of the entire night sky, yet they were sufficient to reveal two previously unknown companions of the Milky Way.

The discoveries offer an encouraging indication of what could emerge as Rubin continues collecting data across a much larger portion of the sky.

"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 of astronomy and physics in Yale's Faculty of Arts and Sciences, and co-author on both studies. "It is remarkable this is now a reality."

The research involved a broad 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 Instituto de Astrofísica de Andalucía in Spain, the University of Colorado-Boulder, and Lund University in Sweden.

Aashay Pai of the University of Chicago served as the first author of the study describing Rubin-GC1.

Hidden Galaxies Could Reveal New Clues About Dark Matter

Finding additional satellites could do much more than expand the known population of galaxies surrounding the Milky Way. These faint objects may also help astronomers investigate one of the universe's greatest mysteries: dark matter.

Dark matter is an invisible form of matter that does not emit or reflect light, making it impossible to observe directly with conventional telescopes. Scientists infer its existence from the gravitational effects it produces 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 particularly valuable for studying dark matter. They contain only a small fraction of the stars found in larger galaxies, yet dark matter dominates their total mass. By examining their properties and movements, researchers can test ideas about how dark matter behaves and influences the formation of galaxies.

These 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 systems.

Rubin's early findings may also provide important clues about how the universe's smallest and faintest galaxies form, evolve, and survive.

"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."