In Webb’s images, the faint red speck called Capotauro moved by just 0.13 arcseconds—but that tiny shift changed its story from a possible glimpse of the universe’s first galaxies to a cold object in our own Milky Way.
The James Webb Space Telescope first spotted Capotauro in infrared light. It showed up in some longer-wavelength images but disappeared at shorter wavelengths, a pattern that can signal an extremely distant galaxy. Such galaxies are valuable because their light lets astronomers study the universe when it was very young. But the same colors can come from nearby objects, so the team needed another way to tell what Capotauro was.
The distant-galaxy idea rested on a sharp cutoff in the object’s light. Hydrogen gas can absorb light below a certain wavelength. As the universe expands, light from a faraway galaxy stretches, shifting that cutoff toward infrared wavelengths. Based on this pattern, Capotauro’s colors suggested a redshift of about 32, which would place it roughly 100 million years after the Big Bang.
A different explanation was that Capotauro is a brown dwarf: an object too small to sustain the hydrogen fusion that powers stars. The coldest brown dwarfs, known as Y dwarfs, can have atmospheres rich in water, methane and ammonia. These molecules absorb infrared light in ways that can create a pattern similar to the cutoff from a distant galaxy. Color alone could not settle the question.
F.-Y. F. Liu of the University of Edinburgh and colleagues tested Capotauro’s position in Webb images taken in December 2022 and June 2026. They lined up the images using 68 compact reference sources and added artificial objects to the images to check how background noise might affect their measurements. Capotauro had shifted about 0.13 arcseconds, with a statistical significance greater than six sigma—a measure showing the result is unlikely to be a random fluctuation.
A galaxy so far away would appear essentially fixed against the background during that time. A member of the Milky Way, by contrast, can move across the sky enough to reveal its nearer location. The researchers also compared Capotauro’s infrared brightness with known brown dwarfs. The best match points to a temperature of about 350 Kelvin, or 77°C, and a distance of roughly 2,400 light-years. That makes it cold for a star-like object, though not cold by everyday standards.
The distance estimate is still uncertain. The known objects used for comparison do not cover the coldest possibilities, so Capotauro could be colder, fainter and closer. The researchers call it one of the most distant Y dwarfs found to date. Another observation, made at a carefully chosen time, could use Webb’s changing viewpoint to help measure its distance more precisely.
For astronomers searching for the earliest galaxies, Capotauro is a useful reminder that unusual colors can have more than one explanation. Returning to the same patch of sky gave researchers a simple but powerful test: the ancient-looking object had moved, and its place was much closer to home.