Rubin Observatory Spots Two Faint Companions of the Milky Way in Early Data

The Vera C. Rubin Observatory has spotted two previously unknown companions of the Milky Way: a probable dim dwarf galaxy and a faint globular cluster. Universe Today reported that both turned up in the observatory's Early Data Preview 2, released in July 2026, and astronomers say they are likely the first of many such finds.

The two objects are named Aquarius IV and Rubin-GC1. Each is described in its own paper, and the lead researcher on the first says the observatory's full 10-year survey should uncover many more.

Two small objects, two different papers

Astronomers have found about 65 dwarf satellite galaxies and about 40 globular clusters around the Milky Way, according to Universe Today, and they suspect many more remain hidden because the objects are so dim.

Aquarius IV is described in Research Notes of the American Astronomical Society. Its lead author, William Cerny, is a graduate student in astronomy at Yale. The paper calls it the first ultra-faint Milky Way satellite identified with Rubin data, and says it was confirmed in images from the Dark Energy Camera. It is a compact, low-luminosity system in the outer halo of the galaxy, made of ancient stars with few heavy elements.

Rubin-GC1 is described in a paper accepted by The Astrophysical Journal Letters and available on arXiv. Its lead author is Aashay Pai, a graduate student in physics at the University of Chicago. The cluster's stars are about 9.2 billion years old and moderately rich in metals. The authors conclude it is most likely a very faint globular cluster, probably pulled into the Milky Way along with debris from the Sagittarius dwarf spheroidal galaxy.

Why telling a dwarf galaxy from a cluster is hard

Ultra-faint dwarf galaxies can contain as few as a few hundred stars, fewer than some globular clusters hold. The key difference is that dwarf galaxies contain dark matter, while globular clusters generally do not, with a few possible exceptions.

That is why Aquarius IV is only a likely dwarf galaxy for now. Its properties closely resemble those of the smallest confirmed ultra-faint dwarfs, the authors write, but they do not rule out a globular cluster. Only a small number of its stars have been detected so far, so settling the question will take deeper imaging and spectroscopy.

Rubin-GC1 was harder to see for a different reason. It lies toward the galactic bulge, where stars are packed densely and dust is thick. Universe Today called finding it there particularly impressive.

Why it matters for the hunt for dark matter

Small satellites like these are leftovers from the way the Milky Way grew through mergers. Cerny said the new discoveries are only the start, and that if Rubin can find such faint systems in preview data, its full survey will almost certainly turn up many more.

Those satellites, he said, will give astronomers new chances to test models of galaxy formation, trace the Milky Way's assembly history and study the nature of dark matter. Marla Geha, a Yale professor of astronomy and physics who co-authored both studies, said that finding new satellites in Rubin data had been an exciting prospect for most of her career, and that it is remarkable to see it become reality.

For general readers, the result is mainly a sign of what the observatory can do. Nothing about the Milky Way has changed, but the census of its faint neighbors, which shapes how astronomers think about dark matter, is now growing again.

What comes next

Deeper observations of Aquarius IV will be needed to decide whether it is a dwarf galaxy or a cluster. Rubin's full 10-year survey is expected, according to the researchers, to add further Milky Way satellites to the list.

Around Prague, based on reporting by Universe Today