Deep in the cold, dusty spaces between stars where no planets exist and no sunlight shines, astronomers have uncovered a chemical giant that shouldn’t be there, but is. Scientists have detected the largest sulfur-bearing molecule ever found in space.
This ring-shaped compound called 2,5-cyclohexadiene-1-thione (C₆H₆S) is found in a massive molecular cloud near the center of the Milky Way.
This discovery is quite important as it fills a long-standing gap in astrochemistry and strengthens the idea that the raw ingredients for life begin forming long before stars, planets, or Earth itself exist.
“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,” Mitsunori Araki, lead researcher and a scientist at the Max Planck Institute for Extraterrestrial Physics (MPE), said.
Finding the missing sulfur in space
Sulfur is a key element in proteins and enzymes on Earth, but complex sulfur molecules had stubbornly refused to show up in space—until now. For instance, scientists had detected many complex carbon-based molecules in space, but sulfur compounds remained surprisingly small and simple, usually made of six atoms or fewer.
This was strange because meteorites and comets in our solar system contain much more complex sulfur chemistry. So somewhere between interstellar space and planetary systems, sulfur seemed to go missing.
To decode this mystery, the researchers decided that instead of guessing what sulfur molecules might exist in space, they would first create and study one in the lab.
They started with thiophenol, a strong-smelling liquid that contains sulfur. By firing a 1,000-volt electrical discharge through it, they broke the molecules apart and allowed them to recombine into new forms—mimicking the energetic conditions found in space. One of the products was C₆H₆S, a stable molecule with a six-membered carbon ring and 13 atoms in total.
The next step was identification. Molecules in space are detected not by sight, but by the radio waves they emit as they rotate. Using a custom-built spectrometer, the study authors measured the exact radio frequencies of C₆H₆S with extreme precision—accurate to more than seven significant digits. This created a unique radio fingerprint.
They then searched for this fingerprint in astronomical data collected using two powerful radio telescopes in Spain—the IRAM 30-meter and Yebes 40-meter telescopes. The signal was there, hiding in plain sight, within a well-studied molecular cloud called G+0.693–0.027, located about 27,000 light-years from Earth, close to the Milky Way’s center.
More importantly, this cloud has not yet formed stars, showing that such chemistry happens very early in cosmic history.
“Our results show that a 13-atom (sulfur) 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,” Valerio Lattanzi, one of the study authors and a scientist at MPE, said.
A lot of sulfur is still missing
This discovery does more than add a new molecule to a cosmic list. It creates a direct chemical bridge between interstellar space and the material found in comets and meteorites, suggesting that complex sulfur chemistry survives the journey from star-forming clouds to planetary systems.
However, detecting one molecule does not yet explain how widespread such chemistry is, or how these compounds evolve as stars and planets form. Sulfur chemistry is notoriously difficult to model, and many sulfur-bearing molecules may still be invisible because their laboratory fingerprints are unknown.
Further research will focus on searching for even larger and more complex sulfur compounds, both in the lab and in space.
The study is published in the journal Nature Astronomy.