Rubin Observatory discovers two hidden companions of the Milky Way. Hundreds more may be waiting
The Vera C. Rubin Observatory has already uncovered two previously unknown companions of the Milky Way using preliminary observations covering just 7% of the night sky. The discoveries include an extremely faint galaxy 355,000 light-years away and a star cluster hidden behind cosmic dust. Scientists suspect hundreds of additional satellite galaxies remain undiscovered, and Rubin's powerful telescope could finally…
Key points
- Rubin Observatory found two faint Milky Way companions, Aquarius IV and Rubin-GC1, using only preview data.
- Aquarius IV lies about 355,000 light-years from Earth; Rubin-GC1 is a star cluster hidden by cosmic dust.
- The preview images covered only about 7% of the night sky, and Rubin's 10-year survey began in June.
The Vera C. Rubin Observatory in Chile has discovered two previously unknown companions of the Milky Way, using observations taken before its official launch. The findings suggest many more faint objects may be found once the observatory's full survey gets underway.
The first object, Aquarius IV, is a tiny galaxy about 355,000 light-years from Earth on the distant outskirts of the Milky Way. The second, Rubin-GC1, is a star cluster on the opposite side of the galaxy, hidden in a region where thick cosmic dust makes observation difficult.
Small share of sky surveyed
The discoveries came from preview observations gathered between April 2025 and January 2026, covering only about 7% of the night sky. Astronomers have identified roughly 65 satellite galaxies around the Milky Way, but some estimates suggest hundreds more remain undiscovered.
Rubin began its 10-year Legacy Survey of Space and Time in June. The observatory uses the 8.4-meter Simonyi Survey Telescope and a 3.2-gigapixel camera, and is jointly funded by the National Science Foundation and the U.S. Department of Energy's Office of Science.
What I find especially exciting is that these discoveries are just the beginning for Rubin
— William Cerny
What researchers said
William Cerny, a Yale astronomy student and lead author of the Aquarius IV study, said the discoveries are only a start. He said the full survey will likely yield many more Milky Way satellites, offering chances to test galaxy formation models and study dark matter.
Marla Geha, a Yale professor and co-author on both studies, said finding such satellites in Rubin data had long been a prospect for her. The research involved institutions including the University of Chicago, the University of Virginia and Duke University.
