Astronomers using NASA’s James Webb Space Telescope (JWST) may have discovered a powerful stellar explosion that occurred when the universe was still in its early stages. The possible supernova, named AT 2026abck, appears to have been observed just four to 10 days after the star exploded, offering scientists a rare opportunity to study the earliest moments of a supernova in the distant universe.
The discovery is particularly exciting because the event lies at a redshift of 5.017, meaning its light has traveled through space for billions of years before reaching Earth. At that time, the universe was much younger than it is today, and galaxies were still actively forming stars.
Reported by a research team led by Ming-Yang Zhuang of the University of Illinois Urbana-Champaign, the finding could help scientists better understand how massive stars lived and died during the universe’s early history.
The research paper was published on the arXiv preprint server on September 29, 2026. However, the object has not yet been definitively classified as a supernova, and further observations are needed to confirm its nature.
Why Finding an Early-Universe Supernova Is Difficult
Supernovae are among the most powerful explosions in the universe. They occur when stars reach the end of their lives, releasing enormous amounts of energy and spreading elements into surrounding space.
Studying these explosions helps astronomers understand how stars evolve, how galaxies develop and how many of the chemical elements found throughout the cosmos are produced and distributed.
However, detecting supernovae in the early universe is extremely challenging.
As light travels across the expanding universe, its wavelength stretches. This process, known as cosmological redshift, shifts the light from distant objects toward longer wavelengths. Light that originally came from the ultraviolet or visible part of the spectrum can therefore arrive at Earth as infrared radiation.
This makes powerful infrared observatories essential for studying the distant universe.
The James Webb Space Telescope was designed to observe infrared light with extraordinary sensitivity. Since beginning scientific operations, it has helped astronomers identify more than 200 potential supernova events in galaxies with redshifts extending to around 6.
Despite these advances, many distant supernova candidates have been observed only once or a few times. Scientists often lack the observations needed to determine exactly when the stars exploded and how their brightness changed immediately afterward.
AT 2026abck stands out because researchers believe they detected it very soon after the explosion.
A Cosmic Explosion That Appeared Between Two Observations
The discovery was made using observations from NEXUS, a JWST Treasury program designed to repeatedly observe the same region of the sky.
On May 28, 2026, the location where AT 2026abck would later appear showed no detectable source at the relevant position. Approximately 71 days later, a bright new object was visible.
The transient reached an apparent magnitude of approximately 27.4 in Webb’s F200W infrared filter. Astronomers measured a brightening of more than 3.6 times compared with the earlier observation.
The observations allowed the team to estimate when the explosion occurred.
Because of the universe’s expansion, events at high redshift appear to unfold more slowly to distant observers. After accounting for this time-stretching effect, the researchers estimated that the explosion occurred approximately four to 10 days before it was discovered.
If confirmed, this would make AT 2026abck an unusually young supernova candidate at such an early cosmic epoch.
Another important detail is that the event was detected without gravitational lensing.
Gravitational lensing occurs when the gravity of a massive object, such as a galaxy or galaxy cluster, bends and magnifies light from an object behind it. This natural magnification can make otherwise invisible distant objects detectable.
At redshifts greater than 5, the only previously confirmed supernova based on its spectrum required gravitational magnification of up to 30 times.
AT 2026abck was found in a field that does not show this type of magnification, making the observation especially valuable.
An Extremely Hot and Luminous Object
The researchers used models to estimate the transient’s physical properties.
Their analysis suggests that AT 2026abck had a temperature of approximately 17,000 Kelvin. This is considerably hotter than the surface temperature of the Sun.
The object’s estimated radius was around 46 times the distance between Earth and the Sun. Its luminosity, a measure of the total energy it emits per second, was approximately ergs per second.
These estimates suggest that the object was exceptionally hot and bright at such an early stage.
Its location also provides important clues. AT 2026abck lies between two components of its host galaxy, approximately 3,600 and 4,200 light-years from the galaxy’s center.
This position makes certain alternative explanations less likely. For example, some powerful transient events occur when a black hole disrupts a star or when an active galactic nucleus produces a sudden flare. Such events are generally associated with a galaxy’s central region.
The location of AT 2026abck argues against these explanations, although it does not completely rule them out.
The observations also make a typical Type Ia supernova less likely because the event’s early ultraviolet brightness, temperature and luminosity do not fit that scenario particularly well.
Could It Be an Unusual Type II Supernova?
One of the leading possibilities is that AT 2026abck is a Type II supernova.
Type II supernovae occur when massive stars reach the end of their lives and their cores collapse under gravity. The resulting explosion ejects the star’s outer layers into space.
These events are important because massive stars help enrich the universe with heavier elements that later become part of new stars, planets and other cosmic structures.
However, AT 2026abck appears unusually energetic compared with typical Type II supernovae observed nearby.
According to the research team, a comparison with 34 well-observed nearby Type II supernovae found that none exceeded approximately 15,000 Kelvin beyond six days after explosion or reached a comparable luminosity after just four days.
AT 2026abck appears to be both hotter and more luminous at a similar stage.
Scientists suggest that additional energy may be powering the event. One possibility is shock cooling, in which material heated by the explosion gradually releases energy as it expands and cools.
Another possibility is an interaction between the expanding supernova material and dense gas surrounding the original star. Such interactions can convert kinetic energy into radiation, making the explosion brighter than an ordinary supernova.
The researchers also consider the possibility that the event is a superluminous supernova, an unusually bright class of stellar explosion.
What Its Host Galaxy Reveals
The galaxy hosting AT 2026abck offers another piece of the puzzle.
Researchers estimate that it contains approximately six billion solar masses of stars and has a star formation rate of around 14 to 21 solar masses per year.
Its metallicity, which describes the abundance of elements heavier than hydrogen and helium, is estimated to be about 30 percent of the Sun’s metallicity.
These properties are broadly consistent with a galaxy in the early universe, when star formation was vigorous and many galaxies contained fewer heavy elements than modern galaxies.
The host differs significantly from the galaxy associated with SN Eos, the only other spectroscopically confirmed supernova at a similar redshift. Comparing these systems could eventually help astronomers understand how stellar explosions varied across different early galaxies.
What Happens Next?
Despite the promising findings, one major limitation remains: scientists do not yet have a spectrum of AT 2026abck itself.
The team’s analysis is based on broadband photometry, which measures an object’s brightness through several filters. These measurements can help researchers estimate its temperature, luminosity and other properties, but they cannot provide a definitive classification in this case.
The redshift of 5.017 was determined from the spectrum of the host galaxy rather than the transient itself.
To establish the object’s true identity, astronomers need deeper observations using Webb’s Near Infrared Spectrograph, known as NIRSpec.
Future observations could also reveal how the object’s brightness changes over time. If it is a Type II supernova, its ultraviolet emission may fade relatively quickly. If it is a superluminous supernova, it could continue brightening.
These changes would help scientists distinguish between the competing explanations.
A New Window Into the Universe’s Past
The possible discovery of AT 2026abck demonstrates how the James Webb Space Telescope is transforming our understanding of the early universe.
By detecting a luminous transient without gravitational magnification and potentially observing it only days after its explosion, astronomers may have found a rare opportunity to investigate how massive stars ended their lives when the universe was young.
If future observations confirm the event as a supernova, it could provide valuable evidence about the behavior of massive stars, the environments in which they formed and the physical processes that powered their final explosions.
For now, AT 2026abck remains a promising candidate rather than a confirmed supernova. But its unusual brightness, high temperature and extremely young estimated age make it an important target for further investigation.
With more observations, Webb could help scientists uncover what happened during one of the universe’s earliest generations of stellar explosions.
Reference: Ming-Yang Zhuang et al., NEXUS: Discovery of a Luminous Young Transient at z=5, arXiv (2026). DOI: 10.48550/arxiv.2609.38373.
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