A look at the dwarf planet Pluto and Titan, Saturn’s largest moon, baffled astronomers after the James Webb Space Telescope (JWST) detected a chemical signature on their surfaces that does not match any recorded in spectroscopic databases. Researchers believe this is not an instrument error, but rather the signature of a compound whose identity remains a mystery—a mixture of materials never studied in a laboratory, or even a compound whose chemistry has not yet been characterized.

The finding appears in a study awaiting publication in the journal Astronomy & Astrophysics. Scientists identified an absorption band centered at 5.113 micrometers on both Titan and Pluto—two worlds separated by billions of kilometers and with very different physical conditions. The signal appeared in observations made with two different instruments on the JWST, leading the team to rule out the possibility that it was a calibration issue or some other type of technical error.

The key to the discovery lies in a technique known as spectroscopy. Each element or molecule interacts with light in a unique way, absorbing certain wavelengths and leaving a characteristic pattern, like a fingerprint. For d...