Astronomers using test data from the Vera C. Rubin Observatory in Chile have discovered a new satellite galaxy of the Milky Way, named Aquarius IV. This galaxy is among the faintest, lightest, and farthest satellite galaxies ever observed, located about 359,000 light-years from the Milky Way's center. The discovery was made using early test data from the observatory, known as Early Data Preview 2 (EDP2), which captured about 7% of the night sky between April 2025 and January 2026. This test data was used to fine-tune the observatory's systems before its full operational phase begins. Aquarius IV is described as a cluster of ancient stars, with an estimated age of about 13 billion years—nearly as old as the universe itself. Its mass is estimated to be about 1,500 times that of the Sun, making it significantly less massive than the Milky Way, which has a mass of about 1.5 trillion times that of the Sun. Aquarius IV is also extremely dim, with an intrinsic brightness of -1.9, which is 14 million times less bright than the Milky Way. These characteristics make it one of the most challenging galaxies to detect. The discovery was made using computer models that analyzed the EDP2 data to identify groups of stars with similar brightness and color, suggesting they evolved together. Researchers confirmed the galaxy’s existence by comparing it to older images from the Dark Energy Camera on the NSF Victor M. Blanco Telescope in Chile. These older images showed faint stars that had previously gone unnoticed. However, the confirmation from the older images had a lower significance level of six sigma, compared to the eight-sigma significance level of the EDP2 data, which indicates a higher level of confidence in the discovery. The study was published in the Research Notes of the AAS, a journal that publishes non-peer-reviewed work in progress. Aashay Pai, a graduate student in physics at the University of Chicago and co-author of the study, noted that follow-up observations are needed to confirm the object's identity. The Rubin Observatory's advanced 3,200-pixel camera, which is as large as a car, can detect objects up to 100 million times dimmer than those visible to the naked eye. This capability makes the observatory particularly suited for finding ultrafaint dwarf galaxies (UFDGs), which are among the tiniest and faintest galaxies in the universe. These UFDGs are mostly made of dark matter, with 99 to 99.9% of their mass being dark matter and only 0.1 to 1% composed of stars. The discovery of Aquarius IV could help scientists better understand dark matter and the properties of the first galaxies that formed shortly after the Big Bang. The Rubin Observatory is expected to find up to 100 such UFDGs during the Large Synoptic Survey Telescope (LSST) survey, which officially began in June. This survey will create the most detailed time-lapse video of the universe ever recorded, capturing millions of changes in the night sky every night and potentially leading to many new discoveries.