The James Webb Space Telescope has detected an unexplained chemical signature on the surfaces of both Pluto, the dwarf planet, and Titan, Saturn's largest moon, which does not match any compound recorded in existing spectroscopic databases. Researchers have ruled out an instrument fault, suggesting instead that the signal comes from a material never studied in a laboratory, or possibly a compound whose chemistry has not yet been characterised. Because identifying an unknown chemical fingerprint is highly unusual, the puzzle could become a significant new question for planetary science.

The signal is an absorption band centred at 5.113 micrometres, and it appeared through two different JWST instruments, which helped the team rule out a calibration error. Scientists compared it against laboratory spectra of candidate compounds such as acetylene, benzene, ketene and allenes, but none matched exactly. The mystery is deepened by the fact that Titan and Pluto have very different conditions—Titan has a thick nitrogen-methane atmosphere and liquid methane lakes, while Pluto has only a tenuous atmosphere and an icy surface—yet both host complex organic chemistry driven by solar radiation and cosmic rays, which may explain the shared signature. Solving it will require further JWST observations and laboratory work, with hopes also pinned on NASA's forthcoming Dragonfly mission to Titan.