Researchers at the University of Hong Kong (HKU) claim to have solved the mystery of "Unidentified Infrared Emission features" that have been detected in stars, interstellar space, and galaxies. For over two decades, the most commonly accepted theory regarding this phenomenon was that these signatures come from polycyclic aromatic hydrocarbon (PAH) molecules - simple organic molecules made of carbon and hydrogen atoms. Now HKU researchers say the substances generating these signatures are actually complex organic compounds that are made naturally by stars and ejected into interstellar space.

The team of Prof. Sun Kwok and Dr. Yong Zhang used observations taken by the Infrared Space Observatory and the Spitzer Space Telescope of stardust formed in exploding stars called novae to show that the astronomical spectra contain a mixture of aromatic (ring-like) and aliphatic (chain-like) components that cannot be explained by PAH molecules.

A schematic of the typical structure of organic matter in the Universe containing a mixture of ring-like (aromatic) and chain-like (aliphatic) chemical sites and containing about 100 carbon atoms

The researchers say the substances generating these infrared emissions actually have chemical structures that are so complex that their structure resembles those of coal and petroleum. Since coal and petroleum are remnants of ancient life and this type of organic matter was only thought to arise from living organisms, the researchers say this suggests that complex organic compounds can be synthesized in space even when no life forms are present.

Supporting an earlier idea by Kwok that old stars are molecular factories capable of manufacturing organic compounds, they say that not only are stars producing this complex matter on extremely short time scales of weeks, but they are also ejecting it into the general interstellar space in between stars.

"Our work has shown that stars have no problem making complex organic compounds under near-vacuum conditions," says Kwok. "Theoretically, this is impossible, but observationally we can see it happening."

Professor Sun Kwok (left) and Dr Zhang Yong of The University of Hong Kong

As the organic stardust is similar in structure to complex organic compounds found in meteorites, the findings raise the possibility that stars enriched the early solar system with organic compounds. With the Earth being bombarded by comets and meteorites early in its life that could potentially have carried the organic stardust, there is a possibility that the seeds of life on Earth were sown by organic compounds created naturally by stars. If that turns out to be the case, it has obvious implications for the chances of life outside our solar system as the complex organic compounds exist throughout the Universe.

Kwok and Zhang's Paper, Mixed aromatic-aliphatic organic nanoparticles as carriers of unidentified infrared emission features is published this month in the journal Nature.