Webb's 'Earliest Galaxy' Capotauro Moved, and That Makes It a Cold Brown Dwarf

A faint red speck in James Webb Space Telescope images looked like a window onto the universe's first galaxies. A new analysis shows it was never that far away. The object, nicknamed Capotauro, has moved across the sky, and that movement puts it inside our own Milky Way as a cold brown dwarf.

F.-Y. F. Liu of the University of Edinburgh and colleagues report the result in the Monthly Notices of the Royal Astronomical Society. Capotauro showed up in some of Webb's longer-wavelength infrared images and vanished at shorter wavelengths. In a distant galaxy, hydrogen gas absorbs light below a certain wavelength, producing a sharp cutoff, and the expansion of the universe stretches that cutoff into the infrared. Read that way, Capotauro's cutoff implied a redshift of about 32, roughly 100 million years after the Big Bang, an extraordinarily early epoch.

The original study, published in February in Astronomy and Astrophysics, had already floated a rival explanation: a cold object in our own galaxy. Brown dwarfs lack the mass to sustain hydrogen fusion. The coldest, called Y dwarfs, have atmospheres of water, methane and ammonia. Those molecules absorb broad stretches of infrared light, leaving a steep rise in brightness at longer wavelengths that can mimic the signature of a distant galaxy.

To settle it, Liu's team compared Webb images from December 2022 with new observations from June 2026. They aligned the frames using 68 compact reference sources, then planted artificial objects in the real backgrounds to test how noise could distort position measurements. Capotauro had shifted by about 0.13 arcseconds. That is a tiny angle, but the detection exceeds six sigma in statistical significance. A galaxy that far away would look fixed over such an interval, while a nearby object moving through the Milky Way would not.

The team then matched Capotauro's infrared colors against known brown dwarfs. The best fit is a temperature of about 350 kelvin, or 77 degrees Celsius, at a distance of roughly 2,400 light-years. The authors call it "one of the most distant Y dwarfs found to date." The distance is uncertain, though, because the comparison objects do not cover the coldest possibilities. Capotauro could be colder, intrinsically fainter and closer than estimated.

Another well-timed observation could pin down the distance directly, through the apparent shift in position caused by Webb's changing vantage point as it moves along its orbit. That would sharpen the picture of this faint Milky Way resident.

The finding carries a lesson for galaxy hunters. Webb can turn up cold objects that imitate some of the most extreme early-galaxy candidates. When an object's colors leave its identity uncertain, the authors note, returning to the same patch of sky can reveal which sources have moved and which may truly belong to the distant universe. For a field racing to find the first galaxies, that is a cheap and decisive test.