26th September 2014
Complex organic molecules found in the centre of our galaxy
In a star-forming cloud near the galactic core, chemicals have been discovered that are more similar to amino acids – the building blocks of life – than any previous finding. Furthermore, these are present in abundant quantities.
For the first time, astronomers have detected a carbon-bearing molecule with a "branched" structure in interstellar space. Iso-propyl cyanide (i-C3H7CN) was discovered in a giant gas cloud known as Sagittarius B2, a dense region of star formation measuring 150 light years across, located 390 light years from the centre of the Milky Way.
An international team from the Max Planck Institute for Radio Astronomy (Germany), University of Cologne (Germany) and Cornell University (USA) made the discovery using the Atacama Large Millimeter Array (ALMA) in Chile, which became operational in March 2013. Twenty of the observatory's 12m radio telescopes were aligned towards the galactic core – about 27,000 light years from Earth.
"Thanks to the new capabilities offered by ALMA, we were able to perform a full spectral survey toward Sagittarius B2 at wavelengths between 2.7 and 3.6 mm, with sensitivity and spatial resolution ten times greater than our previous survey," says Arnaud Belloche, lead author of the study. "But this took only a tenth of the time."
The carbon atoms in the molecule feature a branched structure that is unlike the straight-chain backbone of other molecules detected so far. This opens a new frontier in the complexity of molecules found within star-forming regions, and bodes well for the presence of amino acids, for which this branched structure is a key characteristic. While various types of molecules have been detected in space, the hydrogen-rich and carbon-bearing (organic) molecules that are most similar to the ones necessary for life on Earth seem to be most common in the gas clouds where new stars are formed.
"Understanding the production of organic material at the early stages of star formation is critical to piecing together the gradual progression from simple molecules to potentially life-bearing chemistry," says Belloche.
The search for organic molecules in space began in the 1960s, and 180 different molecular species have been discovered so far. Each type of molecule emits light at particular wavelengths, in its own characteristic pattern, acting like a fingerprint that allows it to be detected using radio telescopes.
Until now, the organic molecules discovered in star-forming regions have shared one major structural characteristic: they each consist of a "backbone" of carbon atoms that are arranged in a single and more or less straight chain. The new molecule discovered by the team, iso-propyl cyanide, is unique in that its underlying carbon structure branches off in a separate strand.
"This is the first ever interstellar detection of a molecule with a branched carbon backbone," says co-author Holger Müller, University of Cologne.
In addition to its structure, the researchers were also surprised by its abundance in this region – it was found to be plentiful, at almost half the concentration of its straight-chain sister molecule (normal-propyl cyanide), which the team had already detected a few years ago.
Robin Garrod, an astrochemist at Cornell University: "The enormous abundance of iso-propyl cyanide suggests that branched molecules may in fact be the rule, rather than the exception, in the interstellar medium."
"Amino acids identified in meteorites have a composition that suggests they originate in the interstellar medium," adds Belloche. “Although no interstellar amino acids have yet been found, interstellar chemistry may be responsible for the production of a wide range of important complex molecules that eventually find their way to planetary surfaces."
"The detection of iso-propyl cyanide tells us that amino acids could indeed be present in the interstellar medium, because the side-chain structure is a key characteristic of these molecules", says Karl Menten, director at Max Planck. "Amino acids have already been identified in meteorites and we hope to detect them in the interstellar medium in the future."