What Astrochemistry is Telling Us

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This image shows the Rho Ophiuchi region of star formation where methyl isocyanate was detected.  The insert shows the molecular structure of this chemical, an important precursor for life’s chemical building blocks. ESO/Digitized Sky Survey 2/L. Calçada

Sometimes lost in the discussion of exoplanets and habitability is where the potential building blocks of life might come from and how they got there.

Yes, hydrogen and water and methane and carbon and nitrogen have been found in abundance around the cosmos, but how about the larger and more esoteric compounds needed for life to emerge?  The precursor compounds to amino acids and nucleobases, for instance. Are they formed in space, too.

Some have indeed been identified around young stars or in star-formation regions, but much of what we know about complex molecules in space comes via meteorites and comets.

The Philae lander, for instance, identified 16 organic compounds on the Churyumov-Gerasimenko comet in 2015, including four never-before detected on comets. Some of these compounds play a key role in the prebiotic synthesis of amino acids, sugars and nucleobases — the ingredients for life.

Now an additional and significant precursor compound has been detected around sun-like stars in the very early stage of their formation.  The chemical is methyl isocyanate, and it is an important building block of life.

The detection was made by two teams at the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope, high in the Chilean desert.  The researchers described their detection as the first one of this prebiotic molecule around a solar-type protostar, the type from which our solar system evolved.

“We are particularly excited about the result because these protostars are very similar to the Sun at the beginning of its lifetime, with the sort of conditions that are well suited for Earth-sized planets to form,” said Rafael Martín-Doménech of the Centro de Astrobiología in Madrid and Víctor M. Rivilla of the Osservatorio Astrofisico di Arcetri in Florence. They were lead authors of one of the two papers published on the subject by the Royal Astronomical Society.

“By finding prebiotic molecules in this study, we may now have another piece of the puzzle in understanding how life came about on our planet.”

The Atacama Large Millimeter/submillimeter Array (ALMA) is a partnership between nations in Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is the largest ground-based astronomical observatory in existence, and it is located on one of the driest spots on Earth. ALMA (ESO/NAOJ/NRAO)

The precursor compound was detected around IRAS 16293-2422, a triple protostar system consisting of a binary star (A1/A2) separated by a distance 47 times the distance from Earth to our Sun.  The far removed third star (B) is 750 times that Earth-Sun distance. IRAS 16293-2422 is around 400 light-years away in a large star-forming region called Rho Ophiuchi in the constellation of Ophiuchus.

Of paramount importance to the researchers is the finding that all of three IRAS stars have masses similar to that of the sun.  And all three were found to have the methyl isocyanate around them.

What’s more, astronomers using the ALMA array found also glycolaldehyde — a simple form of sugar — in the gas surrounding the same stars in 2012.

This discovery was the first time that a sugar had been found in space around a solar-type star in the region where a planet-forming disk is expected to arise — roughly corresponding to the distance between the Sun and Uranus.

Both the discovery of the sugar, and now of the methyl isocyanate,  place these chemical building blocks of life in the right place and at the right time to become part of planets that might be forming around the stars.

Earth and the other planets form from the material left over after the formation of the their host star. So studying solar-type protostars can therefore open a window to the past for astronomers and allow them to observe conditions similar to those that led to the formation of our solar system over 4.5 billion years ago.

Authors Niels Ligterink of Leiden Observatory and Audrey Coutens of University college of London had this to say about the discoveries:  “This star system seems to keep on giving!  Following the discovery of sugars, we’ve now found methyl isocyanate.

This family of organic molecules is involved in the synthesis of peptides  and amino acids which, in the form of proteins, are the biological basis for life as we know it.”

Ironically, while the compound can be an important precursor for life, it is also a very toxic substance.  Indeed, it was the main cause of death following the Bhopal industrial accident  in 1984.

In astrochemistry, a complex organic molecule is defined as consisting of six or more atoms, where at least one of the atoms is carbon. Methyl isocyanate contains carbon, hydrogen, nitrogen and oxygen atoms in the chemical configuration CH3NCO. Artist rendering by L. Calçada.

As described in a European Southern Observatory release about the papers, “ALMA’s capabilities allowed both teams to observe the molecule at several different and characteristic wavelengths across the radio spectrum. They found the unique chemical fingerprints located in the warm, dense inner regions of the cocoon of dust and gas surrounding young stars in their earliest stages of evolution.

“Each team identified and isolated the signatures of the complex organic molecule methyl isocyanate. They then followed this up with computer chemical modeling and laboratory experiments to refine their understanding of the molecule’s origin.”

Having some of the chemical ingredients and precursor ingredients of life present as planets are formed certainly doesn’t mean that life necessarily emerged there.  The same is true if those ingredients are delivered right to the planet surface via meteorite, comet or interstellar dust.

But as scientists work to put together an understanding of how life started on Earth and whether it might exist elsewhere, having some of the same important-for-life compounds present here and in areas where exoplanets form is intriguing for sure.

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