Two teams of researchers report today that they have detected a prebiotic molecule—a potential building block of life—around newly formed sun-like stars. The molecule, methyl isocyanate, has a structure that is chemically similar to a peptide bond, which is what holds amino acids together in proteins.
The finding suggests that quite complex organic molecules may be created very early in the evolution of star systems.
“It shows the level of complexity you can get to before planets form is pretty high. A lot of [spectral] lines were detected which gives confidence that it’s real. It’s a safe detection.” says astrochemist Karin Oberg of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was not involved in the studies.
Methyl isocyanate has become a target for astrochemists ever since the European Space Agency’s Rosetta mission detected the molecule on the comet 67P/Churyumov-Gerasimenko 2 years ago. Comets are thought to have survived unchanged since the early days of the solar system, so the discovery of methyl isocyanate suggested it had been present on the comet since then and didn’t form on a planet.
Although the detection on 67P/Churyumov-Gerasimenko is now questioned by some, methyl isocyanate was also detected in two star-forming clouds, Orion KL and Sagittarius B2(N), in 2015 and 2016, but these are hot environments full of very massive stars, very unlike the situation of the early Sun. Undaunted, researchers began to study more sun-like sources. One group was already surveying a clutch of very young stars known as IRAS 16293-2422.
“We thought, why not look [for methyl isocyanate] in our source,” says team member Niels Ligterink of Leiden Observatory in the Netherlands.
The instrument of choice for such studies is the Atacama Large Millimeter/Submillimeter Array (ALMA), a collection of 66 dishes high in the Chilean Andes. ALMA focuses on the region of the spectrum between radio waves and infrared light, the range of frequencies at which complex molecules emit light when they undergo various transitions. Because the molecules are so complex, there are many possible transitions, each emitting photons of a specific frequency.
So a molecule such as methyl isocyanate will emit a characteristic fingerprint of photons that will appear as spikes or lines in the spectrum detected from the gas cloud. The challenge for astronomers is to identify that fingerprint among the forest of spectral lines from all the other chemicals in the cloud.
Ligterink’s team combed through data they had collected from IRAS 16293-2422 using ALMA in 2014 and 2015 and found 43 clearly identifiable lines from methyl isocyanate. The other team, led by Rafael Martín-Doménech of the Center for Astrobiology (INTA-CSIC) in Madrid, Spain, used new and archived data to find another eight lines in a different frequency range. The two teams report their results in the latest issue of the Monthly Notices of the Royal Astronomical Society.
Both teams then tried to figure out how methyl isocyanate might have formed in such a very cold and inhospitable environment. In a protoplanetary cloud around a young star there would be tiny grains of rocky material that would provide a surface on which chemicals could react.
Ligterink says his team filled a vacuum chamber with a gas mixture of isocyanic acid and methane and chilled it to 15 kelvin, freezing the gases onto a gold surface. They then illuminated the surface with intense ultraviolet light, as you might get from a young sun-like star. The infrared spectrum of the resulting gas, as well as mass spectrometry, showed clear signs of methyl isocyanate.
In space, many other molecules would also be present, says Ligterink, such as water, carbon monoxide, and carbon dioxide. More experiments are needed to ensure these would not block the reaction.
Martín-Doménech says that their chemical modeling of the cloud suggests that reactions on dust grains would not have produced enough of the molecules.
“There must be more reactions in the gas phase to get the abundances we observed,” he says.
Oberg says that although methyl isocyanate is not the most complex organic molecule that’s been detected in star-forming clouds, it’s interesting because it is so similar to a key part of proteins. But she warns about jumping to the conclusion that newly formed planets are seeded with the entire key ingredient for life.
“We don’t know the chemical process. We don’t know if methyl isocyanate is crucial and we don’t know how peptides form,” she says. Although such studies are finding ever more complex molecules ahead of planet formation, “the link to how life forms on planets is unknown.”
The two teams and others will continue to scour gas clouds for other complex organic molecules to fill out the picture of life formation. Martín-Doménech says the holy grail for these searches is amino acids, in particular the simplest one: glycine. Amino acids, the building blocks of proteins, are more complex than the likes of methyl isocyanate and likely to exist in smaller quantities that would be harder to detect. So glycine would be a big prize. Says Martín-Doménech:
“Then you are just one step from proteins.”