Webb’s farthest galaxy candidate Capotauro is a failed star inside the Milky Way

Capotauro, the faint red dot that James Webb Space Telescope images once offered as the most distant galaxy ever seen, has moved. Over three and a half years it slid a tiny distance across the sky, and no object billions of light-years away can do that.

The motion ends a debate that ran for more than a year. Capotauro is a brown dwarf, a failed star too small to burn hydrogen, drifting inside our own Milky Way at roughly the temperature of a freshly made cup of tea. Instead of rewriting the story of the first galaxies, it becomes one of the coldest and most distant objects of its kind ever identified.

The result comes from a team led by Fengyuan Liu and Derek McLeod at the University of Edinburgh’s Institute for Astronomy, and it is now published in Monthly Notices of the Royal Astronomical Society. They measured a shift of 132 milliarcseconds, give or take 20, which rules out a galaxy, an active black hole or a supernova at more than six sigma. Physicists usually need five sigma to claim a discovery.

The stakes were high. Had Capotauro been a galaxy, its light would have left it about 90 million years after the Big Bang, nearly 200 million years earlier than MoM-z14, the most distant galaxy confirmed with a spectrum. It also looked far too bright for such an early object. “If it’s a galaxy with a redshift of 32, many things that we have thought so far would be wrong,” Nicha Leethochawalit of the National Astronomical Research Institute of Thailand said when the candidate was announced.

How a shift the width of a hair settled it

Distant galaxies stay pinned to the same spot on the sky for far longer than a human lifetime. Stars and brown dwarfs inside the Milky Way orbit the galaxy at their own speeds, so over a few years they creep against the background. That drift, called proper motion, is the cleanest way to tell a neighbour from a cosmic giant.

The team compared the CEERS survey discovery images with new ones taken three and a half years later by two Webb programmes, MINERVA and SPAM. Both used filters at the long-wavelength end of Webb’s NIRCam camera, where Capotauro is brightest. The researchers pinned its position to 68 compact background sources and ran 50,000 trials, including fake sources injected into the images, to see how far a measurement like theirs could wobble by chance.

The shift they found, 132 milliarcseconds, equals the width of a human hair seen from more than 100 metres away. Part of an apparent shift can come from parallax, the small yearly wobble caused by Earth circling the Sun, but here it accounts for only about 3 milliarcseconds and points in a different direction. Nearly all of the movement is real motion through space, about 38 milliarcseconds per year.

A spectrum could not settle it: Webb’s faint NIRSpec spectrum fit both readings, and Capotauro is too dim for a better one. So the team turned chemistry into geometry. A galaxy at the edge of time cannot move. A nearby brown dwarf must.

Why a failed star can pass for a newborn galaxy

Astronomers find the earliest galaxies with a trick called the dropout method. The expanding universe stretches light, and hydrogen gas between galaxies absorbs everything bluer than a certain wavelength, so a very distant galaxy vanishes in Webb’s shorter filters and reappears only in the reddest ones. Capotauro showed exactly that pattern, with a drop of about three magnitudes, and the colours pointed to a redshift of about 32.

The coldest brown dwarfs, known as Y dwarfs, can fake the same signature. Water, methane and ammonia in their atmospheres soak up almost all their light at shorter infrared wavelengths, and their glow rises sharply between 4 and 5 microns. Through a handful of broad filters, a dim Y dwarf and an impossibly early galaxy can look alike.

The trap is old. Cool brown dwarfs already posed as galaxies at redshifts of 5 to 7 in the Hubble era; now the impostors are colder Y dwarfs, often missing from the template libraries astronomers used. Two other candidates beyond redshift 30, near the Bullet Cluster, turned out to be Y dwarfs once a second round of Webb imaging caught them moving.

Giovanni Gandolfi of the University of Padua, who led the team that reported the object, called Capotauro a possible “farthest galaxy ever seen” but also listed a cold brown dwarf or a rogue planet among the alternatives. An analysis by the JADES team had already found its colours fit a Y dwarf at about 350 kelvin.

What Capotauro actually is

The new measurements point to a brown dwarf of spectral type Y1, at about 350 kelvin, or 77 degrees Celsius. It gives off roughly one five-millionth of the Sun’s energy and sits about 730 parsecs away, close to 2,400 light-years, with an uncertainty of around 360 light-years.

That makes it an outlier: most known Y dwarfs lie within a few tens of light-years of the Sun, and WISE 0855, the coldest known brown dwarf, is 7.4 light-years away. Only a deep survey built to hunt early galaxies could pick up one this faint and far.

Its speed stands out too: about 132 kilometres per second sideways, roughly 475,000 kilometres per hour. A Milky Way simulation along that line of sight found such speeds in only 0.4% of thin-disc stars but in 8% of the thick disc, an older population aged 10 to 12 billion years, so Capotauro may be an ancient object. Y dwarfs typically weigh 3 to 25 times as much as Jupiter, with temperatures like those of the coldest giant exoplanets photographed so far.

What the measurement doesn’t settle

The distance comes from comparing Capotauro’s colours with templates of known brown dwarfs, not from a direct geometric parallax. The available templates stop at type Y1, so the team cannot rule out an even colder object. If Capotauro is colder and fainter than assumed, it is also closer, which would lower its speed and make it an ordinary member of the thin disc.

Its mass is unknown, and so is most of its chemistry. Capotauro has only been detected between about 4 and 5 microns and is too faint for practical follow-up with Webb’s spectrographs, so the team expects it to serve mainly as a statistical probe of distant brown dwarfs.

Another group had proposed that Capotauro was a pair-instability supernova, the explosion of a giant early star, at a redshift near 15, citing an apparent brightening of about 23%. The Edinburgh team found no significant change between the two sets of images, though a variation of up to 10%, common in brown dwarfs, cannot be excluded. They argue the original brightness measurements were likely about 50% too high because of an oversized measuring aperture. A moving object also rules the supernova out.

The result does not cancel every claim of galaxies beyond the current record. It sets a rule for testing them: candidates at redshifts near 30 should be checked against cold brown dwarf templates, and their positions compared whenever images from more than one epoch exist.

Common questions about Capotauro

What is Capotauro?

Capotauro is a faint infrared source in James Webb Space Telescope images of the CEERS survey field, first proposed as a galaxy at redshift 32, about 90 million years after the Big Bang. Proper motion now shows it is a cold brown dwarf inside the Milky Way. Its discoverers at the University of Padua named it after the old name of Corno alle Scale, a peak in Italy’s Apennines.

What is a Y dwarf?

A Y dwarf is the coldest class of brown dwarf, an object heavier than a planet but too light to fuse hydrogen like a star. Y dwarfs are cooler than about 500 kelvin, some close to room temperature, and water, methane and ammonia in their atmospheres absorb much of their light.

How do astronomers tell a brown dwarf from a distant galaxy?

Colours alone can mislead, because a cold brown dwarf mimics the light drop used to find early galaxies. The cleanest test is to image the object again years later: a distant galaxy stays fixed, while a brown dwarf inside the Milky Way shifts.

What is the most distant galaxy confirmed so far?

The most distant galaxy confirmed with a spectrum is MoM-z14, seen about 280 million years after the Big Bang. Several candidates based only on colours claim earlier times, and Capotauro was one of the most extreme.

The discovery images were taken in December 2022, and the second set came from Webb observations in mid-to-late June 2026. Monthly Notices of the Royal Astronomical Society published the analysis on 28 October 2026. The next step the researchers name is a third round of imaging timed to measure Capotauro’s parallax, which would turn its estimated distance into a measured one, plus wider Webb surveys to catch brighter cousins that are easier to study.

Reference: 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

Tags: astronomy, James Webb Space Telescope, Milky Way, brown dwarfs, Capotauro, CEERS