The James Webb Space Telescope has unmasked a pair of cosmic impostors lurking within our own galaxy: two ultra-cool brown dwarfs that had been mistaken for some of the most distant galaxies ever observed.
The discovery, published in an upcoming issue of the Astrophysical Journal Letters (arXiv:2604.23668), serves as a cautionary tale for astronomers hunting the universe’s first stars and galaxies. It also delivered two of the coldest, lowest-mass brown dwarfs ever spectroscopically confirmed by JWST.
“Astronomers never trust imaging only,” co-author Marusa Bradac of the University of Ljubljana told Sky & Telescope.
A tantalizing deception
The objects, now named Bullet-BD1 and Bullet-BD2, were initially identified during a JWST survey of the Bullet Cluster, a massive galaxy cluster 3.8 billion light-years away in the constellation Carina. Using the dropout technique (a standard method where astronomers flag objects that vanish at certain wavelengths as potential ultra-high-redshift galaxies), both candidates appeared exceptionally promising.
One candidate seemed to drop out of JWST’s shorter-wavelength filters in a pattern indicating a redshift far beyond 14, corresponding to roughly 150 to 200 million years after the Big Bang. In fact, one of the two appeared even more distant than Capotauro, the current photometric record-holder.
If confirmed, either object would have ranked among the earliest galaxies ever seen.
But JWST did something its predecessors could not: it followed up.
Spectroscopy reveals the truth
In March 2025, the team obtained NIRSpec spectroscopy of both candidates. The spectra did not match distant galaxies. Instead, they aligned perfectly with brown dwarf templates: failed stars too small to sustain hydrogen fusion, glowing faintly in infrared from residual heat of formation.
New imaging in January 2026 clinched the case. Both objects showed measurable proper motion across the sky: 49 milliarcseconds per year for Bullet-BD1 and 24 milliarcseconds per year for Bullet-BD2, at 6-sigma and 8-sigma significance respectively. No distant galaxy would show detectable motion over human timescales.
Bullet-BD1 has an estimated temperature of about 350 Kelvin (roughly 80 degrees Celsius, similar to a warm spring day), making it one of the lowest-temperature brown dwarfs ever confirmed. Bullet-BD2 is slightly warmer at about 410 Kelvin.
Both lie between 1,000 and 2,800 light-years from Earth, squarely within the Milky Way.
A growing contamination problem
The discovery highlights a known but underappreciated challenge in early-universe astronomy: the color degeneracy between cold brown dwarfs and high-redshift galaxies in broadband near-infrared filters. Both populations appear faint at short wavelengths and bright at long ones, exactly the same photometric signal that JWST uses to identify its most distant targets.
Marco Castellano of the Astronomical Observatory of Rome, who was not involved in the study, told Sky & Telescope that the findings “highlight an important source of contamination in searches for the earliest galaxies: Cold dwarf stars may mimic the colors and brightness of extremely distant objects more often than previously appreciated.”
Pablo Perez-Gonzalez of the Spanish National Research Council added that Bullet-BD1 and BD2 are “tens to hundreds of times brighter” than many other early-galaxy candidates, meaning it remains unclear how many additional candidates may be similarly contaminated.
A 2025 paper by Tu et al. (arXiv:2510.02026) using more than 40,000 JWST/NIRSpec spectra had already flagged this concern, identifying 68 brown dwarfs across JWST’s extragalactic survey fields.
Serendipitous science
The silver lining is that JWST’s pursuit of the earliest galaxies is also producing a windfall of brown dwarf discoveries. About 3,000 brown dwarfs are currently known, with thousands more awaiting confirmation. Each serendipitous detection adds to astronomers’ understanding of the Milky Way’s population of these elusive objects.
“Sometimes you look for big, splashy galaxies at the edge of the observable universe,” astronomer Phil Plait wrote in Scientific American, “and what you actually find are a pair of equally splashy brown dwarfs in your own backyard.”
The lesson for future surveys is clear: photometric selection alone is not enough. JWST’s spectroscopic follow-up capability, combined with the serendipitous detection of proper motion, turned what would have been a contaminant into a genuine discovery.
The two brown dwarfs will not trouble future galaxy hunters once their positions and motions are cataloged. But they serve as a reminder that even the most powerful telescope ever built can be fooled, and that the universe’s secrets often reveal themselves in unexpected ways.