The Vera C. Rubin Observatory has revealed two previously unknown companions of the Milky Way in preliminary observations covering only about 7% of the night sky. The discoveries offer an early example of how the observatory may help find faint objects that have been difficult to detect—and give astronomers new targets for investigating the history and composition of our galaxy’s surroundings.
The two objects are different kinds of companions. Aquarius IV is an ultra-faint galaxy about 355,000 light-years from Earth, in the distant outskirts of the Milky Way. Rubin-GC1 is a star cluster on the galaxy’s opposite side, where thick clouds of cosmic dust make observations especially challenging.
Researchers found the objects in preview observations gathered between April 2025 and January 2026, before the observatory’s official launch. The report says Aquarius IV was barely detectable by eye in a sky map but was identified with high confidence by automated search algorithms. Rubin-GC1 appeared as a collection of resolved stars. Finding both with a small portion of the expected survey data suggests Rubin can detect some exceptionally faint members of the Milky Way’s neighborhood.
The work is described in two studies. William Cerny, an astronomy student at Yale, led the Aquarius IV study and was a second author on the Rubin-GC1 study. Aashay Pai of the University of Chicago was first author of the Rubin-GC1 study, and Yale professor Marla Geha coauthored both. The Aquarius IV paper appeared in Research Notes of the AAS; the Rubin-GC1 paper had been accepted by The Astrophysical Journal Letters, according to the report. The ScienceDaily story drew on materials provided by Yale University and was originally written by Jim Shelton.
The discoveries matter partly because satellite galaxies help researchers examine how the Milky Way assembled over billions of years. Their positions, motions and other properties can preserve clues about how a larger galaxy accumulated matter and interacted with smaller systems. The new objects add to the known population, but the report does not describe them as resolving the Milky Way’s formation history.
Ultra-faint galaxies may also help astronomers investigate dark matter. They contain far fewer stars than larger galaxies, while dark matter is thought to dominate their total mass. Since dark matter cannot be observed directly with conventional telescopes, scientists infer it from gravitational effects. Studying the properties and motions of ultra-faint systems could let researchers test ideas about how dark matter behaves and how it affects galaxy formation. The discoveries provide potential new subjects for that work; they are not themselves a measurement that settles the nature of dark matter.
Astronomers have identified about 65 satellite galaxies around the Milky Way, while some estimates suggest hundreds more may remain undiscovered. Those estimates point to the scale of the search, not a confirmed count of hidden galaxies. Rubin began its planned 10-year Legacy Survey of Space and Time in June. It will repeatedly photograph large areas of the Southern Hemisphere sky using an 8.4-meter telescope and a 3.2-gigapixel camera, building a record of the changing sky.
Cerny said the early discoveries raise expectations for the longer survey, which could find many more Milky Way satellites. Geha described the findings as a step toward completing the census of satellite galaxies, a goal that could help explain how the faintest galaxies form. How many additional companions Rubin will find remains uncertain; the early results show what a small preview can reveal, while the full survey will provide a much broader search.