Astronomers have identified a real sugar molecule — not just a simple sugar-like precursor — drifting in the cold, dark space between stars. An international team led by researchers from the Center for Astrobiology (CAB) in Spain detected erythrulose, a four-carbon monosaccharide, in the molecular cloud G+0.693−0.027 near the center of the Milky Way. The discovery, published in Nature Astronomy, marks the most complex sugar ever confirmed outside our Solar System and offers fresh clues about how the building blocks of life may form before planets even exist.
Erythrulose is a true sugar — technically a ketose with four carbon atoms — that is found naturally on Earth in raspberries and is also used in some self-tanning cosmetic products. While astronomers had previously detected nearly 30 molecules linked to prebiotic chemistry in space, including simple alcohols and aldehydes, none of them qualified as a true sugar. This is the first time a complete monosaccharide has been unambiguously identified in the interstellar medium.
The team used the Yebes 40-meter radio telescope in Spain and the IRAM 30-meter telescope in southern Spain to observe the molecular cloud G+0.693 at millimeter wavelengths. Every molecule has a unique spectral fingerprint — a set of frequencies at which it absorbs or emits radiation as its atoms rotate and vibrate. By scanning the radio signals coming from the cloud and matching them against laboratory spectra of erythrulose, the researchers detected the molecule's' unmistakable signature.
What surprised the team was the absence of simpler three-carbon sugars in the same region. This contradicts the long-held assumption that sugars in space form through a stepwise chain-extension process, where a two-carbon molecule adds one carbon at a time. The presence of a four-carbon sugar without its three-carbon precursors suggests that alternative, non-sequential chemical pathways may be at work in interstellar space — a finding that could reshape theories of prebiotic chemistry.
The origin-of-life connection. Sugars are essential biomolecules. Ribose, a five-carbon sugar, is the backbone of RNA, widely believed to have been a key player in the earliest forms of life. Finding that complex sugars can form in interstellar clouds — before stars and planets exist — supports the hypothesis that some of life's raw materials were delivered to early Earth via comets, asteroids, and interstellar dust, rather than forming exclusively on the planet's surface.
A window into prebiotic chemistry. The detection of erythrulose opens the door to searching for even more complex sugars, including ribose itself, in space. If ribose can form under interstellar conditions, it would strengthen the case that the molecular inventory for life is widespread throughout the galaxy.
Testing models of chemical complexity. The absence of three-carbon sugars challenges existing models of interstellar chemistry. Understanding which organic molecules can form in space — and which cannot — helps scientists reconstruct the chemical evolution that eventually led to life on Earth and potentially on other worlds.