The NSF–DOE Vera C. Rubin Observatory has not finished its first year of science operations, yet astronomers combing through its test images have already found two previously unknown objects orbiting the Milky Way. One, named Aquarius IV, appears to be an ultra-faint dwarf galaxy. The other, Rubin-GC1, is a small star cluster tucked behind thick dust on the far side of our galaxy, according to a University of Washington announcement released Nov. 5.

Both objects came from preview data collected between April 2025 and January 2026. That dataset covers only about 15% of the sky area the telescope will eventually survey, the university said. If such a small slice of test images already holds new neighbors, the full survey could substantially redraw the map of the Milky Way's surroundings.

The discovery is also an early return on a major U.S. science investment. Rubin is funded by the U.S. National Science Foundation and the Department of Energy's Office of Science, and its 10-year Legacy Survey of Space and Time (LSST) officially began in June.

Tiny Galaxies Built Almost Entirely of Dark Matter

Ultra-faint dwarf galaxies are the smallest and dimmest galaxies known. According to Live Science, they contain at most a few hundred thousand stars, compared with roughly 100 billion in the Milky Way. Joshua Simon of the Carnegie Observatories, who was not involved in the research, told the outlet that dark matter makes up 99% to 99.9% of their mass.

That imbalance makes them natural laboratories. Dark matter cannot be seen directly, yet it accounts for about 85% of all matter in the universe. Models of how dark matter clumps predict how many small satellite galaxies should surround a galaxy like ours, so counting them is one of the most direct ways to test those models.

Their stars are also ancient. EarthSky reports that the stars in Aquarius IV are poor in heavy elements, suggesting they formed about 13 billion years ago. That turns these faint systems into fossils — survivors from the era of the universe's first galaxies.

Aquarius IV and Rubin-GC1 by the Numbers

Aquarius IV lies roughly 355,000 light-years from Earth in the Milky Way's outer halo, in the direction of the constellation Aquarius. Its absolute magnitude of -1.9 makes it about 14 million times dimmer than the Milky Way. Aashay Pai, a University of Chicago graduate student and co-author, called it "one of the faintest and most compact UFDGs at a large distance."

The team detected the star grouping at a statistical significance of 8 sigma, meaning the odds of it being a random collection of stars are vanishingly small. Archival images from the Dark Energy Camera on the Victor M. Blanco Telescope in Chile then confirmed faint stars that earlier surveys had overlooked. The Aquarius IV paper, led by Yale graduate student William Cerny, appeared in Research Notes of the American Astronomical Society.

Rubin-GC1 is a different kind of object. It was published Oct. 28 in The Astrophysical Journal Letters and sits in a dust-obscured region on the far side of the galaxy. Universe Today reports that the cluster lies toward the constellation Ophiuchus, is about 9.2 billion years old, and was likely pulled into the Milky Way from a smaller satellite.

A Satellite Census That Could Grow Sharply

About 65 satellites of the Milky Way are currently known, according to the University of Washington. Earlier modeling work cited by Universe Today predicted a total population of roughly 100 to 300 satellite galaxies, meaning most may still be undiscovered. Pai told Live Science he expects Rubin to find up to 100 more during the LSST.

The Aquarius IV paper is direct about the opportunity. "Rubin LSST thus stands poised to revolutionize the census of ultra-faint Milky Way satellites," the authors wrote. They estimate that systems similar to Aquarius IV will be detectable with more than 85% efficiency as the survey's sensitivity improves.

Peter Ferguson, a University of Washington postdoctoral scholar and co-author of both studies, said finding objects this faint depends heavily on a clean catalog that separates true stars from distant galaxies. More than 20 UW researchers worked on focusing images, validating data, and maintaining that catalog. "Aquarius IV and Rubin-GC1 are a preview of what the next decade will look like," Ferguson said.

The evidence still has limits. Live Science notes that Research Notes of the AAS publishes work in progress without traditional peer review, and EarthSky reports that the team considers the dwarf-galaxy label preliminary. Aquarius IV could still turn out to be an unusually large globular cluster. Only spectroscopic measurements of its stars' motions can show whether a heavy dark matter halo is holding it together. Rubin-GC1, by contrast, has passed peer review.

The Decade-Long Survey Ahead

Rubin sits atop Cerro Pachón in northern Chile and carries a 3.2-gigapixel camera — the largest digital camera ever built for astronomy. It photographs the southern sky every few nights, and repeated exposures stack into deeper images over time. That means objects even fainter than Aquarius IV should gradually emerge from the data.

The next steps fall to follow-up observatories. Large telescopes will need to measure the speeds of individual stars in Aquarius IV to confirm its nature and weigh its dark matter. Meanwhile, the Rubin team will keep processing survey data, and Yale's Marla Geha said continued discoveries will help complete the census of satellite galaxies.

Backyard observers should not expect to see these objects. At an apparent magnitude of 18.3, Aquarius IV is far beyond the reach of typical amateur telescopes. Readers who want to follow along can track announcements through the Rubin Observatory and its citizen science programs, which invite volunteers to help classify survey data.

The bottom line is that two new Milky Way companions emerged from a sample covering only a fraction of Rubin's planned map. One classification remains preliminary, but astronomers involved in the work expect many more faint neighbors as the 10-year survey builds. Each one adds a data point to one of the biggest open questions in physics: what dark matter is and how it shaped the galaxies we see today.

What Readers Want to Know

What did the Rubin Observatory find? Astronomers found Aquarius IV, a likely ultra-faint dwarf galaxy, and Rubin-GC1, a faint star cluster hidden behind dust. Both orbit the Milky Way and were spotted in preview data gathered before the main survey began.

How far away is Aquarius IV? It lies about 355,000 light-years from Earth in the Milky Way's outer halo. Live Science puts its distance at roughly 359,000 light-years from the galaxy's center.

Why do ultra-faint dwarf galaxies matter for dark matter research? They are made almost entirely of dark matter and contain very old stars. Counting them and measuring their mass helps scientists test models of how dark matter behaves.

Is Aquarius IV confirmed as a galaxy? Not yet. The team calls the classification preliminary, and follow-up measurements of its stars' motions are needed to rule out a large star cluster.

How many more satellites could Rubin find? About 65 Milky Way satellites are known. Co-author Aashay Pai expects Rubin could find up to 100 more over its 10-year survey.

Can I see Aquarius IV with a telescope at home? No. It is far too faint for typical amateur equipment. Updates and citizen science opportunities are available through the Rubin Observatory.