Rubin finds a pair of new satellites around our galaxy
· BehindLines newsroom
The first data from the Vera C. Rubin Observatory in Chile have revealed two new objects orbiting the Milky Way: Aquarius IV, probably a very faint dwarf galaxy, and Rubin-GC1, most likely a globular cluster. According to Universe Today, both discoveries come from the "Early Data Preview 2" dataset, released in July 2026.
Aquarius IV, a dwarf galaxy yet to be confirmed
According to Universe Today, Aquarius IV is a compact, low-luminosity system located in the outer halo of the galaxy, with a population of ancient stars that are poor in heavy elements. It would be the first satellite of this type identified using Rubin data. According to EarthSky, it lies about 355,000 light-years from Earth, and the team confirmed it in archival images from the Dark Energy Camera mounted on a telescope at Cerro Tololo.
The nature of the object remains uncertain: Universe Today and EarthSky specify that a very large globular cluster is not ruled out. Universe Today explains that only a few stars have been detected and that deeper images and spectroscopy will be needed to settle the question. The distinction matters, because dwarf galaxies contain dark matter, which is generally not the case for globular clusters. The study, led by William Cerny of Yale University, appeared in Research Notes of the AAS.
Rubin-GC1, a cluster hidden by dust
Still according to Universe Today, Rubin-GC1 lies in the direction of the galactic bulge, a region dense with stars and rich in dust. Its stars would be about 9.2 billion years old and moderately rich in heavy elements. The object probably comes from the Sagittarius dwarf galaxy and is linked to a stream of stars called the Sagittarius leading arm. The work, led by Aashay Pai of the University of Chicago, has been accepted by The Astrophysical Journal Letters. Universe Magazine says that Italy's National Institute for Astrophysics relayed these results.
What this changes
According to Universe Today, about 65 satellite dwarf galaxies and some forty globular clusters are known in the halo, but astronomers suspect that many remain to be discovered. These objects preserve the trace of the Milky Way's past mergers; mapping them can show where dark matter clumps together. According to EarthSky, the data used date from a test phase, before the start of the major ten-year survey launched in June 2026. The authors expect, according to Universe Today, to detect this type of object with an efficiency above 85% over the course of the survey.