Vera Rubin Observatory Spots Two Hidden Companions of the Milky Way

Using early data from the Vera C. Rubin Observatory, astronomers have identified two previously unknown satellites of the Milky Way: a likely ultra-faint dwarf galaxy named Aquarius IV (Rubin J2201−0234), found in the outer galactic halo, and a globular cluster called Rubin-GC1 (Rubin J1628−1735), detected in the direction of the constellation Ophiuchus near the galactic bulge. Both were spotted in Early Data Preview 2 (EDP2), released in July 2026.

Aquarius IV is ancient, dim, and low in heavy elements — properties that closely resemble the smallest ultra-faint dwarf galaxies. However, the researchers note that a globular cluster classification has not been ruled out, and confirming its true nature will require deeper imaging and spectroscopy. Rubin-GC1 is about 9.2 billion years old and moderately metal-rich, making it most likely a faint globular cluster associated with the Sagittarius Dwarf Spheroidal Galaxy's leading tidal arm. Its detection is especially notable given the dense stars and thick dust in the direction of the galactic bulge.

Lead author William Cerny, a graduate student at Yale, said the full 10-year Legacy Survey of Space and Time will "undoubtedly provide us with many more Milky Way satellite discoveries." Co-author Marla Geha, a professor of astronomy and physics at Yale, called it remarkable that such finds are now a reality. Together, these discoveries are expected to sharpen understanding of galaxy formation, the Milky Way's assembly history, and the nature of dark matter.