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Scientists have discovered the largest organic molecule containing sulfur — a key ingredient for life — ever identified in interstellar space. The researchers call this finding a “missing link” in the scientific understanding of the cosmic origin of the chemistry of life.
Sulfur is the tenth most abundant element in the universe and a fundamental component of amino acids, proteins and enzymes on Earth. However, although researchers had previously found sulfur-containing molecules similar to the newly discovered one in comets and meteorites, there was a puzzling absence of large sulfur-containing molecules in interstellar space, a vast region between stars dotted with clouds of dust and gas.
“Sulfur came to Earth from space a long, long time ago,” said Mitsunori Araki, a scientist at the Max Planck Institute for Extraterrestrial Physics in Germany and lead author of a study on the discovery, published last week in the journal Nature Astronomy.
“However, we have only found a very limited number of sulfur-bearing molecules in space, which is strange. It should exist in large quantities, but it is very difficult to find.”
Another team of researchers previously suggested that sulfur might appear scarce in space because it is trapped in cosmic ice, hidden in plain sight rather than absent.
Therefore, the new detection adds an important piece to this puzzle. “This is the largest sulfur-bearing molecule ever found in space, with 13 atoms,” Araki said. “Before this, the largest one only had nine atoms, but it was already a rare case, because most of the sulfur molecules detected only had three, four or five atoms.”
Finding larger molecules is important, he added, because it helps fill a gap between the simple chemistry found in space and the more complex building blocks of life discovered in comets and meteorites.
The molecule, which also contains carbon and hydrogen, is called 2,5-cyclohexadiene-1-thione and joins a growing catalog of more than 300 molecules observed in space so far. The finding, Araki said, suggests that many more, perhaps even larger, sulfur-containing molecules could be detected in the future.
The molecule was found in a molecular cloud called G+0.693–0.027, about 27,000 light years from Earth, near the center of our galaxy.
Molecular clouds are cold, dense concentrations of dust and gas that allow the formation of molecules. They act as stellar nurseries, as gravity creates clusters that eventually become young stars.
“A molecular cloud is where star formation occurs,” said Valerio Lattanzi, also a scientist at the Max Planck Institute for Extraterrestrial Physics and co-author of the study.
Lattanzi added that, over time, some of these clouds will lead to the formation of planetary systems like our own solar system. “The ingredients that are embedded in the molecular cloud will be transferred to the planets,” he said. “We’re trying to figure out what the ingredients are that will eventually make up life, trying to understand how from simple molecules we get to life as we know it on Earth. And we’re trying to add elements to this picture, one by one.”
The researchers first synthesized the molecule by electrically shocking a substance called thiophenol—an unpleasant-smelling liquid containing sulfur, carbon, and hydrogen. They then obtained an extremely precise “radio fingerprint” of the molecule, which they compared with cloud-observing telescope data collected by the IRAM-30m and Yebes radio telescopes in Spain.
“We have seen in previous observations that sulfur molecules were quite abundant in this cloud,” Lattanzi said. “That’s why it was a very good goal for us. We think that one of the possible origins of life on Earth is through collisions and impacts of small bodies like comets and meteorites with our planet in the past, which probably brought complex molecules, including some containing sulfur. So that’s what we’re trying to do: connect these missing links along the way to eventually form life as we know it.”
Kate Freeman, professor of geosciences at Pennsylvania State University, called the study “an exciting detective story made possible by powerful radio telescopes and a really good search strategy.”
Meteorites are known to contain large, complex sulfur compounds, said Freeman, who was not involved in the research, and many of them likely came to Earth to help lay the foundations for the chemistry of life.
“However, we didn’t really know how these compounds ended up in meteorites or their precursor planetary bodies,” he added in an email. “We now know that there is at least a high probability that some of them come from outside the solar system, from molecular-rich regions of our galaxy, such as the area studied by the authors.”
Sulfur is one of the six elements essential for life on Earth and may have been a key ingredient in early life on Earth, providing vital fuel to ancient microbes, according to Sara Russell, professor of planetary sciences at the Natural History Museum in London.
“The presence of complex organic molecules at the center of our Milky Way galaxy implies that biologically important materials may be everywhere in space,” Russell, who was not involved in the study, said in an email. “Finding such molecules so far from our home planet also suggests that similar processes may be occurring elsewhere, which increases the likelihood that life exists on another planet.”
Ryan Fortenberry, an associate professor of chemistry and biochemistry at the University of Mississippi, said he was excited by the findings. “Sulfur as an atom occupies a very special place in the periodic table. It has a unique chemistry that allows molecules to do much more than oxygen, nitrogen and carbon alone would allow,” Fortenberry said by email. “Finding sulfur-containing molecules helps us better measure where life might have started and where it might evolve.”
More than 50 years ago, finding any molecule in space was a miracle, added Fortenberry, who was also not involved in the study.
“The common thinking was that the hostile environment would simply break them down; now we are finding molecules with 13 atoms and some with several dozen atoms,” he said.
“Molecules are more resistant than we thought, and telescopes have shown us that the chemistry of space is much richer than we could imagine. I am convinced that we will find amino acids in space, beyond our solar system.”
