A faint red object spotted by the James Webb Space Telescope originally appeared to offer a possible glimpse of the universe’s first galaxies. But images taken about 3½ years apart revealed a giveaway: The object, nicknamed Capotauro, had moved.
That shift places it much closer to home than previously assumed. Capotauro is actually a cold brown dwarf in our Milky Way, rather than a galaxy whose light has traveled across almost the entire history of the universe.
F.-Y. F. Liu of the University of Edinburgh and colleagues report the finding in the Monthly Notices of the Royal Astronomical Society. Their analysis resolves an ambiguity that Capotauro’s unusual colors had left open at first sighting.
How a cold object mimics an ancient galaxy
Capotauro appeared in some of Webb’s longer-wavelength infrared images but disappeared at shorter wavelengths. That pattern made it an intriguing candidate for identification as an extremely distant galaxy.
Hydrogen gas absorbs much of an early galaxy’s light below a particular wavelength, creating a sharp cutoff. As that light travels through an expanding universe, its wavelengths stretch, shifting the cutoff into the infrared. Astronomers use an object’s position to estimate how far back in cosmic history they are looking.
Interpreted this way, Capotauro’s cutoff suggested a redshift of about 32. That would place it roughly 100 million years after the Big Bang, an exceptionally early chapter of galaxy formation.
Yet the original study of Capotauro published in February in Astronomy and Astrophysics also considered a very different explanation: that it is a cold object within our own galaxy.
Brown dwarfs lack enough mass to sustain the hydrogen fusion that powers stars. The coldest varieties, called Y dwarfs, have atmospheres that contain water, methane and ammonia. These molecules absorb broad ranges of infrared light, leaving a steep rise in brightness at longer wavelengths that can resemble the distant-galaxy cutoff.
Colors alone could therefore be misleading. Movement offered a different test.
The giveaway was its position
Liu’s team compared Webb images from December 2022 with observations taken in June 2026. They aligned the images using 68 compact reference sources, then inserted artificial objects into the real backgrounds to check how noise could distort their position measurements.
Capotauro had shifted by about 0.13 arcseconds—a tiny angle, but a detection with a statistical significance exceeding six sigma.
An extremely distant galaxy would appear essentially fixed against the background over that interval, but an object moving within the Milky Way could show just such a shift.
The researchers then compared Capotauro’s infrared brightness pattern with those of known brown dwarfs. The best match suggests a temperature of about 350 Kelvin, or 77°C, and a distance of roughly 2,400 light-years. That is cold for an object resembling a star, although distinctly hot by everyday standards.
At that estimated distance, the authors describe Capotauro as “one of the most distant Y dwarfs found to date.” The estimate remains uncertain, however—the available comparison objects do not cover the coldest possibilities. Capotauro could in fact be colder, intrinsically fainter and closer.
Making another carefully timed observation could help measure its distance through the apparent positional change caused by Webb’s changing viewpoint. That would sharpen the picture of this faint member of the Milky Way.
The reclassification of Capotauro does offer a useful result for galaxy hunters. Webb can uncover cold objects that imitate some of the most extreme early-galaxy candidates. When their colors leave their identification uncertain, returning to the same patch of sky can reveal which objects have moved—and which may really belong to the distant universe.
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Publication details
F-Y F Liu et al., A clear detection of proper motion confirms that the claimed z ≃ 32 galaxy candidate, “Capotauro”, is a Y-type brown dwarf, Monthly Notices of the Royal Astronomical Society (2026). DOI: 10.1093/mnras/stag2055
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A tiny shift exposes a cold brown dwarf masquerading as an early galaxy (2026, November 3)
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