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.
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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
sunkwok@hku.hk
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