Vera Rubin Observatory Uncovers One of Faintest Satellite Galaxies Ever Detected

In a groundbreaking early result from the Vera C. Rubin Observatory in Chile, astronomers have announced the detection of a previously unknown galaxy orbiting the Milky Way. The object, designated Aquarius IV, ranks among the lightest, faintest, and most distant satellite galaxies ever observed. Although the study has not yet undergone peer review, the finding highlights the telescope’s extraordinary sensitivity and expands our understanding of the cosmic neighborhood.

Aquarius IV belongs to a class of objects known as ultrafaint dwarf galaxies (UFDGs), which are among the smallest and dimmest systems in the universe. According to Joshua Simon, an astronomer at the Observatories of the Carnegie Institution for Science who was not involved in the research, these clusters contain at most a few hundred thousand stars, a stark contrast to the Milky Way’s estimated 100 billion stars.

“These galaxies are composed almost entirely of dark matter: 99 to 99.9% of their mass is dark, and only 0.1 to 1% is made up of stars,” Simon explained in an email to Live Science. Due to their diminutive size and low luminosity, even UFDGs in the vicinity of the Milky Way are challenging to detect. While a 2020 study estimated that hundreds of such satellite galaxies should exist, only approximately 60 have been confirmed to date, according to Aashay Pai, a graduate student in physics at the University of Chicago and co-author of the new study.

The Rubin Observatory, perched atop Cerro Pachón mountain in Chile, is equipped with a massive, car-sized camera featuring 3.2 billion pixels. This instrument can detect objects up to 100 million times dimmer than what is visible to the naked eye, making it ideally suited for uncovering UFDGs as it repeatedly scans the night sky every few nights.

The discovery of Aquarius IV, however, did not come from the main Large Synaptic Survey Telescope (LSST) data. Instead, it was identified in Early Data Preview 2 (EDP2), a test run conducted between April 2025 and January 2026 that covered roughly 7% of the sky. Researchers used computer models to analyze this data, looking for groups of stars with brightness and color profiles indicating a common origin. By assuming the stars were approximately 13 billion years old—nearly as old as the universe itself—the team could distinguish the ancient cluster from foreground and background stars.

The findings, published in August in the journal Research Notes of the AAS, reported an eight-sigma significance level, meaning there is only about a 1-in-1.6-quadrillion chance that the observed star group is a random alignment. To verify the discovery, the researchers also examined older images taken by the 570-megapixel Dark Energy Camera on the NSF Victor M. Blanco Telescope in Chile. Those images revealed faint stars previously overlooked, supporting the galaxy’s existence with a six-sigma confidence level, or roughly a 1-in-100-billion chance of being a statistical fluke.

Located in the direction of the constellation Aquarius, Aquarius IV sits approximately 359,000 light-years from the center of the Milky Way, which spans about 100,000 light-years. Pai described it as “one of the faintest and most compact UFDGs at a large distance.” The galaxy’s mass is estimated at roughly 1,500 times that of the sun, compared to the Milky Way’s 1.5 trillion solar masses. With an absolute magnitude of -1.9, it is 14 million times dimmer than our home galaxy.

While Simon termed the discovery robust, Pai cautioned that follow-up observations are necessary to fully confirm the object’s identity. Beyond mapping the nearby universe, studying UFDGs offers critical insights into dark matter, which constitutes about 85% of all matter in the cosmos but does not interact with light. “The stars in ultra-faint dwarfs are also extremely old, dating back to just after the Big Bang, so they give us a unique window to the properties of the first galaxies that formed,” Simon noted.

With the LSST survey officially commencing in June, the Rubin Observatory is expected to discover up to 100 additional UFDGs. The survey aims to produce the most detailed time-lapse video of the universe ever recorded, potentially revealing millions of changes in the night sky each night and leading to countless future discoveries.