Rubin Observatory finds two hidden companions of the Milky Way
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Preview data covering about 7% of the night sky revealed two previously unknown Milky Way satellites, a dwarf galaxy and a star cluster. Astronomers think hundreds more may still be hidden.
The Vera C. Rubin Observatory in Chile has uncovered two previously unknown companions of the Milky Way, giving an early sign of what its survey may achieve. Researchers used preview observations gathered between April 2025 and January 2026, which covered only about 7% of the night sky.
The first object, Aquarius IV, is a tiny galaxy in the distant outskirts of the Milky Way, roughly 355,000 light-years from Earth. The second, Rubin-GC1, is a star cluster on the opposite side of the galaxy, hidden in a region where thick dust clouds make observing difficult.
Astronomers have identified about 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, using an 8.4-meter telescope and the largest digital camera ever built, with 3.2 gigapixels of resolution.
William Cerny, a Yale graduate student and lead author of the Aquarius IV study, said the full survey will likely bring many more discoveries. Marla Geha, a Yale professor and co-author on both studies, called the results incredibly promising. Aashay Pai of the University of Chicago led the Rubin-GC1 study.
Such faint satellites could help researchers test models of galaxy formation, trace the Milky Way's assembly history and investigate dark matter, which dominates the mass of ultra-faint galaxies like Aquarius IV.
