Webb catches a possible supernova at redshift 5, just days after the blast

The James Webb Space Telescope has spotted what may be a supernova in the early universe, caught only about four to ten days after the star exploded. The object, named AT 2026abck, sits in a star-forming galaxy at redshift 5.017. That means we see it as it was roughly a billion years after the Big Bang. Unusually, it was found without help from gravitational lensing.

The result comes from a team led by Ming-Yang Zhuang (University of Illinois Urbana-Champaign). Their paper was posted on arXiv on 29 September and reported by Phys.org on 13 October. It has not yet been peer reviewed.

How it was found

The event turned up in NEXUS, a Webb Treasury programme that revisits the same patch of sky about every two months. On 28 May 2026 the spot was empty. Some 71 days later a new source had appeared at about magnitude 27.4, more than 3.6 times brighter in the F200W filter.

Why it stands out

Webb has found more than 200 possible supernovae out to a redshift of about 6. But beyond redshift 4, most were seen only once or a few times, and very few were caught within days of exploding. At redshift above 5, only one supernova has been confirmed with a spectrum, and that one was magnified up to 30 times by a galaxy cluster.

The team's models give AT 2026abck a temperature of about 17,000 K, a radius of about 46 times the Earth–Sun distance and a luminosity of about 3 × 10⁴³ erg/s. It lies about 3,600 and 4,200 light-years from the centres of two parts of its host galaxy. The authors say this makes a star torn apart by a black hole, or a flare from an active galactic nucleus, unlikely. It is also too hot and too bright in ultraviolet for a Type Ia explosion.

What it could be

None of 34 well-observed nearby Type II supernovae was this hot or this bright at a similar age. The best match is a Type II supernova surrounded by dense material shed by the star, which would add extra early light. A superluminous supernova caught on the rise is also possible.

There is no spectrum of the transient yet. The analysis rests on brightness measured in a few filters, so the team is calling for deep NIRSpec spectroscopy. A Type II event should fade quickly in ultraviolet, while a superluminous one would keep brightening.

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