The Vera C. Rubin Observatory on Cerro Pachón in the Chilean Andes has announced the discovery of two previously unknown companions of the Milky Way. The discoveries were made on the basis of preliminary Early Data Preview 2 data covering about 7% of the sky.
The first object is Aquarius IV, a candidate ultra-faint dwarf galaxy. It is located on the distant outskirts of our Galaxy, roughly 355 thousand light-years from Earth. The properties of Aquarius IV are close to those of the faintest known dwarf galaxies of the Milky Way, but a final classification of the object requires further observations.
The second object is Rubin-GC1, a star cluster. It lies on the opposite side of the Milky Way and is hidden behind dense clouds of interstellar dust. Most likely, it is a faint member of the Galaxy's globular cluster population.
The data were collected from April 2025 to January 2026 as part of the Early Data Preview 2 program. This dataset covered about 3 thousand square degrees — roughly 7% of the entire celestial sphere — and served as a preliminary test before the start of the main ten-year LSST survey.
The analysis was carried out by astronomers from Yale University, the University of Chicago and other scientific institutions. The paper on Aquarius IV has been published in Research Notes of the American Astronomical Society; its lead author is Yale University graduate student William Cerny. The work on Rubin-GC1 has been published in The Astrophysical Journal Letters.
These finds may be only the beginning. The full ten-year Legacy Survey of Space and Time (LSST), researchers expect, will help reveal many more faint and hard-to-observe objects in the vicinity of the Milky Way. It is already clear that Rubin Observatory is capable of finding systems that previously went unnoticed even in difficult, dusty regions of the sky.
