The night sky is like an ocean: the longer you stare, the more faint lights you notice in the depths. That's how astronomers, peering into the outskirts of the Milky Way, found Aquarius IV — a tiny stellar system that had been hiding in the dark until now.

The system was spotted in images from an early survey by the future Vera Rubin Observatory. It is the first such object discovered with its data. Aquarius IV lies 109 kiloparsecs away (about 350 thousand light-years) — far beyond the bright disk of the Galaxy, in the so-called halo. It is quite small: its radius is about 19 parsecs, and its total brightness is comparable to that of a few hundred Suns. For perspective, the distance to it is roughly three times the diameter of the Milky Way itself.

By all signs — low luminosity, compactness, and an ancient, metal-poor stellar population — the find resembles ultra-faint dwarf galaxies. But it could just as well be a globular cluster of stars that wandered to the outskirts. Astronomers currently have too little data to confidently choose between these two options. New, deeper images and spectra of individual stars are needed — then we can figure out what Aquarius IV really is.