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  Discovery of complex organic matter in space

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Prof. Sun Kwok (left) and Dr. Yong Zhang (right) of HKU.

How did life originate on Earth? For over 50 years, scientists believed that life was the result of chemistry involving simple molecules such as methane and ammonia cooking in a primordial soup. In a recent paper published in Nature, Prof. Sun Kwok and Dr. Yong Zhang of the University of Hong Kong showed that old stars are capable of making very complex organic compounds. Using data from the Infrared Space Observatory and the Spitzer Space Telescope, Kwok and Zhang showed that star dust contains significant amounts of organic compounds with aromatic (ring-like) and aliphatic (chainlike) structures. The compounds are so complex that their chemical structures resemble those of coal and petroleum. Since coal and oil are remnants of ancient life, this type of organic matter was thought to arise only from living organisms. The team’s discovery suggests that complex organic compounds can be synthesized in space even when no life forms are present.

The researchers investigated an unsolved phenomenon: a set of infrared emissions detected in stars, interstellar space, and galaxies. These spectral signatures are known as “Unidentified Infrared Emission features”. For over two decades, the most commonly accepted theory on the origin of these signatures has been that they come from simple organic molecules made of carbon and hydrogen atoms, called polycyclic aromatic hydrocarbon (PAH) molecules. Kwok and Zhang showed that the astronomical spectra have features that cannot be explained by PAH molecules. Instead, the team proposes that the substances generating these infrared emissions have chemical structures that are much more complex. By analyzing spectra of star dust formed in exploding stars called novae, they show that stars are making these complex organic compounds on extremely short time scales of weeks.

Due to interference from the Earth atmosphere, infrared observations can only be carried out by telescopes mounted on orbiting satellites. Data of the analysis were obtained with the Short Wavelength Spectrometer instrument of the Infrared Space Observatory of the European Space Agency and the Infrared Spectrograph of the Spitzer Space Telescope of NASA.
Not only are stars producing this complex organic matter, they are also ejecting it into the general interstellar space, the region between stars. The work supports an earlier idea proposed by Kwok that old stars are molecular factories capable of manufacturing organic compounds. “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.”

Most interestingly, this organic star dust is similar in structure to complex organic compounds found in meteorites. Since meteorites are remnants of the early Solar System, the findings raise the possibility that stars enriched the early Solar System with organic compounds. The early Earth was subjected to severe bombardments by comets and asteroids, which potentially could have carried organic star dust. Whether these delivered organic compounds played any role in the development of life on Earth remains an open question.

A spectrum from the Infrared Space Observatory superimposed on an image of the Orion Nebula where these complex organics are found. Credit of the Orion image: NASA, C.R. O’Dell and S.K. Wong (Rice University).

The findings of this research were reported in many international media, including BBC, MSNBC, space.com, Science News, Chemistry World of the Royal Society of Chemistry, and many international newspapers in November 2011.

Prof Sun Kwok
Department of Physics
The University of Hong Kong