JWST spots imposters that mimic distant galaxies

JWST spots – By using the dropout technique and then verifying with deeper spectroscopic follow-up, astronomers have shown that two “extremely distant galaxy” candidates spotted in JWST images of the Bullet Cluster are actually Bullet-BD1 and Bullet-BD2—very low-mass, very

It starts with a red dot.

In the early images from the James Webb Space Telescope. some of those dots looked like they could be galaxies from the very edge of the observable universe—objects so far away that their light had taken an immense amount of time to reach Earth. But the sky, even to powerful telescopes, can play tricks. Most objects in deep space are still too small and distant to resolve much structure. and the universe can appear stubbornly two-dimensional: dim could mean nearby. or dim could mean incredibly distant and intrinsically bright.

That uncertainty is why the breakthrough doesn’t end with a first glance—it depends on how you measure distance. For JWST. that question is central to a dream it launched with in 2021: to clearly observe very young galaxies by looking farther back in time. Light is fast, but the universe is vast. When astronomers peer far enough away, they see galaxies as they were not long after the universe’s birth.

There’s also the catch of cosmic expansion. As the universe expands, it stretches the wavelengths of ancient light through a phenomenon called redshifting. For normal galaxies. visible light can be abundant at emission. but across great distances that visible light arrives shifted into infrared wavelengths. JWST is optimized for infrared observations, pushing its reach well beyond what the Hubble Space Telescope could do.

Even so, the “two-dimensional” look of distant images can still tempt astronomers into mistaking one kind of object for another. One of the tools used to estimate distances quickly is the photometric redshift method. and for extremely remote galaxies there’s a specific approach within it: the dropout technique.

The logic runs through how intergalactic space handles ultraviolet light. Wavelengths in the extreme ultraviolet are absorbed very efficiently by clouds of hydrogen in intergalactic space. blocking most of that UV light from more distant objects. For faraway galaxies, redshifting stretches even that extreme UV light into infrared wavelengths. With the dropout technique, astronomers use a series of filters, each designed to block a different set of wavelengths. A faraway galaxy can appear in the filters corresponding to longer wavelengths. but it will “disappear” in shorter-wavelength filters where its UV emission is faint. The particular filter where the dropout occurs—determined by which filter the galaxy is seen in—lets researchers infer an approximate redshift.

It’s fast, but not perfect. That imprecision helped fuel a wave of early claims based on JWST’s first images. including papers arguing that some candidates were redshifted so far that they pushed cosmological models past their breaking point. The question was unavoidable: were those galaxies truly that distant?.

Answering that required spectra. Confirming redshift demands a time-consuming process of taking candidate galaxies’ spectra, breaking their light into thousands of individual colors. Different elements such as oxygen and hydrogen emit light at very specific wavelengths. Identifying those details allows astronomers to determine an object’s true redshift—and therefore its true distance—with excellent accuracy. In the follow-up spectra for many of the most extreme candidates. the objects turned out to be galaxies much closer to us. with colors that made them look distant only because the dropout method had suggested otherwise.

The most recent example comes from a different target.

In 2025. a team of astronomers used JWST to observe the Bullet Cluster. a galaxy cluster described as relatively near to Earth. In their observations. they applied the dropout technique again—this time to search for extremely distant galaxies that would appear far in the background of the image. In a paper posted in April on the arXiv.org preprint repository. they reported the discovery of two objects called Bullet-BD1 and Bullet-BD2. Both were described as red dots displaying dropouts in the filters that would indicate they are extremely distant. very young galaxies.

Then came the check that the sky demands.

Scientists didn’t stop at the preliminary photometric inference. They conducted deeper spectroscopic observations via JWST and cross-referenced the findings against archival images of the Bullet Cluster. The result was a sharp reversal: the follow-ups showed that Bullet-BD1 and Bullet-BD2 weren’t galaxies at all. They were instead extremely low-mass brown dwarfs located in the Milky Way.

Brown dwarfs sit in a strange middle ground between giant planets and small stars. Astronomers began finding them in the 1990s, and about 3,000 are now known, though thousands more candidates still await confirmation. They’re very faint in visible light, but can be bright in infrared—if they aren’t too far away. That’s exactly the situation for Bullet-BD1 and Bullet-BD2.

What makes these two objects especially valuable is their extremity. They are described as among the lowest-temperature and lowest-mass brown dwarfs known. Bullet-BD1 and Bullet-BD2 have temperatures of about 125 and 27 degrees Celsius—the second number compared to the temperature of a warm spring day. Unlike stars, brown dwarfs don’t have ongoing thermonuclear fusion to generate energy in their cores. After they form, they generally just cool over time.

Because of that cooling history. finding some of the coldest. lowest-mass brown dwarfs matters for how these objects form—a process still debated among researchers. And because Bullet-BD1 and Bullet-BD2 were found by coincidence. the discovery also hints that other. yet-unseen brown dwarfs may be scattered throughout the galaxy. The statistics are still weak because very few such objects are known. making this pair a meaningful addition to what astronomers can currently count and compare.

The payoff is not that the universe failed to deliver distant galaxies. It’s that the imposters themselves turned out to be revealing. Sometimes the sky’s red dots don’t point to the early universe—they point to something equally intriguing much closer to home.

JWST James Webb Space Telescope Bullet Cluster Bullet-BD1 Bullet-BD2 dropout technique photometric redshift brown dwarfs Milky Way redshift infrared astronomy