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 decades, scientists have compiled vast catalogs of these spectral signatures to identify compounds such as water, methane, carbon dioxide, or ammonia on planets and moons, as well as on other bodies outside the solar system.

In this case, the comparison yielded no convincing matches. Furthermore, at this stage, discovering a chemical signature that cannot be linked to a known compound is highly unusual. Therefore, figuring out what is happening on Titan and Pluto could become the new fundamental question for planetary science.

Researchers have already explored several possibilities. They examined laboratory spectra of ices and organic compounds that might exist on these worlds, including acetylene, benzene, ketene, and a family of molecules known as alenes. None of them exactly match the observed signature. The most likely explanation is that it’s from a known compound that exists in a physical state or mixture never before studied in the laboratory, although the authors do not rule out the possibility that the signal comes from a material whose chemistry has not yet been characterized.

The fact that the same signal appears in two such different places makes the mystery even more intriguing. Titan has an atmosphere rich in nitrogen and methane with a surface pressure of approximately 1.5 bar—higher than Earth’s—as well as rivers and lakes of liquid methane and a temperature of about –180 degrees Celsius (–292 Fahrenheit). Pluto, on the other hand, retains only a tenuous atmosphere of about 10 microbars (some 150,000 times less dense); has an ice-covered surface composed of nitrogen, methane, and carbon monoxide; and reaches temperatures close to –235 C (–391 Fahrenheit).