Astronomers have spotted two previously unknown satellites of the Milky Way in the first public trove of data from the new Vera C. Rubin Observatory, adding to the long roster of faint objects orbiting our galaxy.
The first is Aquarius IV, an ultra-faint dwarf galaxy sitting in the Milky Way's distant outskirts. It is so dim that researchers found it not by eye but by running automated search algorithms over the observatory's preview images, flagging a thin, otherwise invisible overdensity of stars. The discovery is described in a paper in Research Notes of the American Astronomical Society, led by Yale University graduate student William Cerny.
The second, Rubin-GC1, is a star cluster on the far side of the Milky Way, tucked inside a dust-choked region near the galaxy's bulge where earlier surveys struggled to see anything at all. It appears in the new data as a dense knot of individually resolved stars, reported separately in The Astrophysical Journal Letters on Oct. 28.
A preview of a decade-long survey
Both finds come from the Simonyi Survey Telescope at the National Science Foundation–Department of Energy Vera C. Rubin Observatory in Chile, built around a 3.2-gigapixel camera — the largest digital camera ever constructed. The two satellites turned up in preview observations taken between April 2025 and January 2026, covering only about 15% of the sky the telescope's 10-year Legacy Survey of Space and Time, which began in June, will eventually map.
"Finding something as faint as Aquarius IV comes down to how clean your stellar catalog is," said Peter Ferguson, a University of Washington postdoctoral researcher and co-author of both studies.
Cerny said the pair of discoveries hints at how much the survey could still turn up once it is running at full scale. "These new satellites will give us new opportunities to test models of galaxy formation," he said. Yale astronomy professor Marla Geha, also a co-author on both papers, said the work is part of a broader effort to inventory everything orbiting the Milky Way: "Continued discoveries will allow us to complete the census of satellite galaxies around the Milky Way."
Astronomers currently know of several dozen confirmed Milky Way satellites, most found only in the last two decades as survey telescopes and computing power improved. Some researchers have estimated that Rubin's full survey could eventually triple that number, giving cosmologists a much larger sample to test predictions about how small galaxies form and survive — or get torn apart — in the gravitational pull of a much larger one.
