In the heart of our galaxy, scientists discovered the first sulfur-bearing six-membered ring molecule hiding in an interstellar cloud. Credit: MPE/ NASA/JPL-Caltech
Researchers at the Max Planck Institute for Extraterrestrial Physics (MPE), in collaboration with astrophysicists from the Centro de Astrobiología (CAB), CSIC-INTA, have identified the largest sulfur-bearing molecule ever found in space: 2,5-cyclohexadiene-1-thione (C₆H₆S). They made this breakthrough by combining laboratory experiments with astronomical observations. The molecule resides in the molecular cloud G+0.693–0.027, about 27,000 light-years from Earth near the center of the Milky Way.
With a stable six-membered ring and a total of 13 atoms, it far exceeds the size of all previously detected sulfur-containing compounds in space. The study is published in Nature Astronomy.
Significance of the discovery for astrochemistry
"This is the first unambiguous detection of a complex, ring-shaped sulfur-containing molecule in interstellar space—and a crucial step toward understanding the chemical link between space and the building blocks of life," says Mitsunori Araki, scientist at MPE and lead author of the study.
"The discovery suggests that many more complex sulfur-bearing molecules likely remain undetected—and that the fundamental ingredients of life may have formed in the depths of interstellar space, long before Earth came into existence."
Until now, astronomers had only detected small sulfur compounds—mostly with six atoms or fewer—in interstellar space. Large, complex sulfur-containing molecules were expected, particularly due to sulfur's essential role in proteins and enzymes, yet these larger molecules had remained elusive. This gap between interstellar chemistry and the organic inventory found in comets and meteorites had been a central mystery in astrochemistry.
The newly discovered C₆H₆S is structurally related to molecules found in extraterrestrial samples—and is the first of its kind definitively detected in space. It establishes a direct chemical "bridge" between the interstellar medium and our own solar system.
How the molecule was detected
The team synthesized the molecule in the lab by applying a 1,000-volt electrical discharge to the evil-smelling liquid thiophenol (C₆H₅SH). Using a self-developed spectrometer, they precisely measured the radio emission frequencies of C₆H₆S, producing a unique "radio fingerprint" with more than seven significant digits. This signature was then matched to astronomical data from a large observational survey led by CAB, collected with the IRAM 30m and the Yebes 40-meter radio telescopes in Spain.
"Our results show that a 13-atom molecule structurally similar to those in comets already exists in a young, starless molecular cloud. This proves that the chemical groundwork for life begins long before stars form," says Valerio Lattanzi, scientist at MPE.
Implications for the origins of life
The discovery suggests that many more complex sulfur-bearing molecules likely remain undetected—and that the fundamental ingredients of life may have formed in the depths of interstellar space, long before Earth came into existence.
In a modern laboratory at Aarhus University and at an international European facility in Hungary (HUN-REN Atomki), researchers Sergio Ioppolo and Alfred Thomas Hopkinson conduct pioneering experiments. Within a small chamber, the two scientists have mimicked the environment found in giant dust clouds thousands of light-years away. This is no easy feat.
The temperature in these regions is a freezing -260° C. There is almost no pressure, meaning the researchers must constantly pump out gas particles to maintain an ultra-high vacuum. They are simulating these conditions to observe how the remaining particles react to radiation, exactly as they would in a real interstellar environment.
The discovery is significant because it suggests that these essential molecules are far more abundant in the universe than previously believed...
"Eventually, these gas clouds collapse into stars and planets. Bit by bit, these tiny building blocks land on rocky planets within a newly formed solar system. If those planets happen to be in the habitable zone, then there is a real probability that life might emerge," Ioppolo explains...
"These molecules are some of the key building blocks of life," explained co-author Professor Liv Hornekær, the InterCat center leader. "They might actively participate in early prebiotic chemistry, catalyzing further reactions that lead toward life."
"That said, we still don't know exactly how life began. But research like ours shows that many of the complex molecules necessary for life are created naturally in space."...
"We've already discovered that many of the building blocks of life are formed out there, and we'll likely find more in the future."
"We already know from earlier experiments that simple amino acids, like glycine, form in interstellar space. But we were interested in discovering if more complex molecules, like peptides, form naturally on the surface of dust grains before those take part in the formation of stars and planets," says Ioppolo.
Peptides are amino acids bonded together in short chains. When peptides bond with one another, they form proteins, which are essential for life as we know it. Looking for the precursors to proteins is therefore vital in the search for the origin of life, Ioppolo explains.
The two researchers placed glycine in the chamber, irradiated it with cosmic ray analogs produced by an ion accelerator at HUN-REN Atomki, and analyzed the results.
"We saw that the glycine molecules started reacting with each other to form peptides and water. This indicates that the same process occurs in interstellar space," Hopkinson says. "This is a step toward proteins being created on dust particles, the same materials that later form rocky planets."
Where stars are born
Ioppolo, Hopkinson, and their colleagues at Aarhus University study and mimic the giant dust clouds between the stars because these are the birthplaces of new solar systems.
"We used to think that only very simple molecules could be created in these clouds. The understanding was that more complex molecules formed much later, once the gases had begun coalescing into a disk that eventually becomes a star," Ioppolo explains. "But we have shown that this is clearly not the case."
The discovery is significant because it suggests that these essential molecules are far more abundant in the universe than previously believed.
"Eventually, these gas clouds collapse into stars and planets. Bit by bit, these tiny building blocks land on rocky planets within a newly formed solar system. If those planets happen to be in the habitable zone, then there is a real probability that life might emerge," Ioppolo explains.
"That said, we still don't know exactly how life began. But research like ours shows that many of the complex molecules necessary for life are created naturally in space."
A universal reaction
It might seem like a minor discovery that peptides form naturally from the simplest amino acids in space. However, the chemical process through which amino acids bond is universal. This suggests that the same reaction likely occurs with other, more complex amino acids as well, explains Hopkinson.
"All types of amino acids bond into peptides through the same reaction. It is therefore very likely that other peptides naturally form in interstellar space as well," says Hopkinson. "We haven't looked into this yet, but we are likely to do so in the future."
Amino acids and peptides are not the only building blocks essential to life; membranes, nucleobases, and nucleotides are necessary as well. Whether these also form naturally in space remains unknown, but Ioppolo, Hopkinson, and their colleagues at the Center for Interstellar Catalysis are working hard to find out.
"These molecules are some of the key building blocks of life," explained co-author Professor Liv Hornekær, the InterCat center leader. "They might actively participate in early prebiotic chemistry, catalyzing further reactions that lead toward life."
"There's still a lot to be discovered, but our research team is working on answering as many of these basic questions as possible," Ioppolo says. "We've already discovered that many of the building blocks of life are formed out there, and we'll likely find more in the future."
