Rubin Observatory spots two hidden companions of the Milky Way
Early images covering only 7% of the night sky have revealed a faint dwarf galaxy and a dust-shrouded star cluster, with astronomers expecting many more to follow.
The Vera C. Rubin Observatory in Chile has found two previously unknown companions of the Milky Way, and astronomers think they are only the first of many. The discoveries came from preview images covering just 7% of the night sky.
The pair are an ultra-faint dwarf galaxy called Aquarius IV and a star cluster named Rubin-GC1. Aquarius IV sits in the distant outskirts of our galaxy, about 355,000 light-years from Earth. Rubin-GC1 lies on the opposite side of the Milky Way, tucked behind thick clouds of cosmic dust that make observing there especially tricky.
Large galaxies are surrounded by smaller satellites, including dwarf galaxies and star clusters. Some contain so few stars that they are fiendishly hard to spot. Around 65 satellite galaxies are currently known around the Milky Way, yet some estimates suggest hundreds more are still waiting to be found.
The observatory stands on Cerro Pachón in the Chilean Andes. It pairs an 8.4-metre telescope with the largest digital camera ever built, which captures images at 3.2 gigapixels. Its 10-year Legacy Survey of Space and Time began in June, and the new finds came from observations taken between April 2025 and January 2026, before that launch.
Two studies describe the work. William Cerny, a Yale graduate student, led the Aquarius IV paper, published in Research Notes of the AAS, and is second author of the Rubin-GC1 paper, which has been accepted by The Astrophysical Journal Letters. Aashay Pai of the University of Chicago led the cluster study, and the teams drew on a wide international collaboration.
Cerny said that if Rubin can find such faint objects in preview data alone, its full survey will “undoubtedly” bring many more. Marla Geha, a Yale professor and co-author on both studies, was equally chuffed. “It is remarkable this is now a reality,” she said.
Why it matters
Faint satellite galaxies are dominated by dark matter, the invisible substance that makes up most of the universe’s matter and which scientists detect only through its gravity. Studying them gives researchers a way to test ideas about how dark matter behaves and how the smallest galaxies form. Their positions and motions also help reveal how the Milky Way was assembled over billions of years. A fuller census of these neighbours would fill in a long-missing piece of our galaxy’s story.
