A team of astronomers scouring the distant universe for ancient objects that formed shortly after the Big Bang stumbled upon an unexpected discovery: a single emission line coming from an unidentified source.

Using the Webb space telescope, scientists detected the unusual glow in a seemingly empty patch of the sky in the southern constellation of Fornax. The light appeared twice over the course of three months, despite earlier observations by both Webb and the Hubble space telescope showing no visible object at that location. Its origin remains a mystery, and the possible candidates aren’t your garden-variety space objects. The findings are detailed in a new paper published in The Astrophysical Journal.

Astrophysicist Mason Huberty of the University of Minnesota was searching for yet-to-be detected objects. Using Webb’s Near InfraRed Spectrograph, Huberty and his team blocked light from known objects such as galaxies that had been previously identified by Hubble. The researchers then searched the data for emission lines.

Six out of seven detections could be matched to distant galaxies. The seventh one, however, left the team puzzled. It appeared as a single bright line during two observations windows, which were carried out 88 days apart in August and November 2023.

The team compared the emission line to possible sources in earlier Webb images but could not pinpoint its origin. Both Hubble and Webb had stared at that corner of the sky before, but nothing could be seen there.

Emission lines are bright spikes of light emitted by atoms or molecules when they release energy.

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