The frigid clouds of gas and dust between stars, what astronomers refer to as the interstellar medium, may seem like an unlikely place for complex chemistry. Temperatures hover near absolute zero, and pressures are magnitudes lower than those on Earth. But although individual chemical reactions occur only rarely, the passage of millions of years has enabled increasingly complex organic molecules to accumulate before the clouds collapse to form stars and planetary systems.
To study those reactions, astronomers observe molecular clouds near the Milky Way’s galactic center, where large columns of gas and dust have created what astrophysicist Izaskun Jiménez-Serra of the Center for Astrobiology in Spain calls “the richest chemical repositories in the Milky Way.”
In a new study that pointed two radio telescopes at a molecular cloud, researchers detected erythrulose, a four-carbon monosaccharide that could have been transferred to minor bodies, such as comets and asteroids, and then delivered to younger planets such as the early Earth (Nat. Astron. 2026, DOI: 10.1038/s41550-026-02905-7). Although astronomers have previously detected nearly 30 molecules linked to prebiotic chemistry, this study reports the first true sugar identified in interstellar space.
“We have previously reported the detection of precursors of ribonucleotides, precursors of protolipids, and precursors of amino acids, but this is the first sugar,” says Jiménez-Serra, who is also the first author of the study.
The detection is so surprising that when chemist Brett McGuire of the Massachusetts Institute of Technology first saw the study’s preprint in early June, he included the results in a talk he was giving at Leiden Observatory in the Netherlands that day “to alert everyone that this had happened.”
“We’ve never managed to detect a sugar [outside of our solar system],” McGuire says. “And now we skipped all the three-carbon sugars and went straight to four carbons. That’s astonishing.”
Although previous detections of glycolaldehyde have garnered attention before, McGuire cataloged the response as perhaps overzealous: “Glycolaldehyde is a diose, so two-carbon, which is not formally, by a chemist’s definition, a sugar.”
Finding a sugar in space
Researchers identified erythrulose, a four-carbon monosaccharide, in the interstellar medium, suggesting that some sugars can form before stars and planets.
The team targeted a giant molecular cloud, G+0.693-0.027, where molecular species that have formed on icy dust grains are released as gas molecules, making them relatively easy to detect. At the cloud’s low temperatures, molecules rotate and emit radio waves at characteristic frequencies that act as molecular fingerprints. By comparing observations with laboratory measurements of erythrulose’s rotational spectrum, the team identified multiple matching spectral lines consistent with the sugar.
Because sugars are difficult to vaporize without decomposing, scientists had to mix erythrulose with talc, which helped release intact gas phase molecules for spectroscopic measurements in the lab. The team then identified and modeled more than 180 molecular species in the observational dataset in order to unambiguously determine which erythrulose signals were sufficiently free of interfering emission signals.
“I was looking at the data without any hope because we hadn’t detected the three-carbon sugars,” Jiménez-Serra says. “Then I started seeing that all the transitions were matching signals present in the observations.”
Rethinking how sugars form
Generally, the larger a molecule is, the more rarely it is expected to form in space. But the four-carbon sugar appears to be at least eight times as abundant as analogous three-carbon sugars.
“The fact that the simpler version is not there means that there’s something really strange, really interesting going on here about the chemistry that we don’t understand yet,” McGuire says.
Researchers involved in the study proposed a formation mechanism different from what astrochemists had previously assumed: instead of growing one carbon atom at a time, erythrulose may form when two abundant two-carbon molecules—glycolaldehyde and ethylene glycol—react on the surfaces of icy interstellar dust grains. The researchers supported the idea with quantum chemical calculations and astrochemical simulations showing that the proposed pathway can produce erythrulose under interstellar conditions.
“This chemistry is weird and unexpected, meaning it goes against our chemical intuition,” McGuire says. “That’s exciting because it opens an entirely new avenue to explore, and it might give us clues for other larger, more complex sugars that we should be looking for. Maybe we should reexamine how we thought about simple chemistry as well.”
“What we know for certain is that these molecules formed during the initial conditions of the process of planetary formation,” Jiménez-Serra says, “even before a star or a planet forms.”
