Rubin Observatory Discovers Aquarius IV 355,000 Light-Years Away, Its First Ultra-Faint Milky Way Satellite
Updated
Updated · EarthSky · Aug 18
Rubin Observatory Discovers Aquarius IV 355,000 Light-Years Away, Its First Ultra-Faint Milky Way Satellite
2 articles · Updated · EarthSky · Aug 18
Summary
Aquarius IV emerged from Rubin Observatory early data as one of the faintest known Milky Way satellites, marking the facility’s first ultra-faint satellite-galaxy discovery.
355,000 light-years from Earth in the Milky Way’s outer halo, the object was identified in Early Data Preview 2 and then checked against archival Dark Energy Camera images from Chile.
Researchers say Aquarius IV is probably an ultra-faint dwarf galaxy—an ancient, metal-poor, dark-matter-dominated class—but they still need spectroscopy to rule out an unusually large globular cluster.
Rubin began its 10-year LSST survey in June 2026, yet this find came from test-phase observations taken between April 2025 and January 2026, underscoring the observatory’s sensitivity early in operations.
With roughly 60 such Milky Way companions known and likely hundreds still undiscovered, scientists say Rubin could detect similar systems with more than 85% efficiency and reshape the satellite census.