For several decades, research in astrochemistry has focused on identifying complex organic molecules in the interstellar medium, considered to be the precursors of the elementary building blocks of life. Until now, these compounds had only been observed inside the Milky Way, mainly in gaseous form or in favorable environments. An international team, led by Marta Sewiło (NASA Goddard Space Flight Center and University of Maryland), has crossed an unprecedented threshold: detecting five of these molecules, frozen on dust grains around a forming star located in the Large Magellanic Cloud.
This neighboring galaxy, poor in heavy elements, offers extreme conditions close to those of the young universe. The study, published on October 20, 2025 in The Astrophysical Journal Letters, questions the chemical mechanisms capable of producing these compounds in much more constrained contexts than previously assumed.
A unique detection of life bricks in an extragalactic environment
The James Webb Space Telescope (JWST), via its Mid-Infrared Instrument (MIRI), enabled the first detection of complex organic molecules in solid form outside the Milky Way. These molecules, called COMs (Complex Organic Molecules), were identified around a young stellar object in formation, designated ST6, located in the Large Magellanic Cloud (LMC). It is a dwarf satellite galaxy of ours, approximately 160,000 light years from Earth.
The team analyzed infrared signatures emitted by the ice surrounding ST6. These light spectra revealed the presence of five carbon-rich molecules: methanol (CH₃OH), ethanol (CH₃CH₂OH), acetaldehyde (CH₃CHO), methyl formate (HCOOCH₃) and acetic acid (CH₃COOH). These compounds are all well known on Earth, with some being used as solvents or in the food industry.
© NASA's Goddard Space Flight Center
Diagram illustrating the volatile organic compounds (VOCs) detected on icy dust grains around ST6: acetaldehyde, acetic acid, ethanol and methyl formate.
The detection of these molecules in an ice phase – and not a gas phase – proves essential. Indeed, it attests to their formation on interstellar dust grains, under very low temperature conditions, close to -250°C. It is also the first time that acetic acid has been detected in solid form in space, all environments combined.
According to lead author Marta Sewiło, it was the unprecedented spectral quality of JWST that made this breakthrough possible. She details in a press release: “ We obtained more information in a single spectrum than in all previous campaigns “. This precision not only makes it possible to identify molecules, but also to quantify their relative abundance.
The Large Magellanic Cloud, a natural laboratory of the cosmic past and the emergence of life?
The Large Magellanic Cloud, an irregular galaxy neighboring the Milky Way, offers a unique astrophysical environment for studying prebiotic chemistry in conditions analogous to those of the young universe. This galaxy is distinguished by a low content of heavy elements — a characteristic called low metallicity. It has about a third to half the amount of carbon, nitrogen and oxygen found in our own galaxy. This means that the atoms necessary for the formation of organic molecules are much less available there.
© NASA's Goddard Space Flight Center/Mr. Sewilo et al. (2025)
Infrared spectrum of the ST6 protostar.
This particularity makes the detection of COMs in the interstellar ice of the LMC all the more surprising. Researchers have shown that despite this chemical deficit, complex molecules can still form. This goes against preconceived ideas according to which a richness in heavy elements is essential for chemical complexification.
The region observed around ST6 is located in a zone of intense star formation. It is located within a structure called superbubble N158, close to the famous Tarantula Nebula. This activity generates strong ultraviolet radiation which tends to destroy fragile molecules. The fact that such complex compounds were able to form and persist there indicates an unexpected robustness of the chemistry mechanisms on dust grains.
Marta Sewiło emphasizes that “the LMC acts like a time capsule”. It makes it possible to probe astrophysical conditions close to those which prevailed a few billion years after the Big Bang. In this sense, these observations provide a window into the capacity of the early universe to generate the fundamental ingredients of organic chemistry. Long before the formation of planetary systems like ours.
Chemistry on interstellar dust grains
The discovery of COMs around ST6 sheds new light on the mechanisms of formation of these complex molecules. Unlike formation in gas, which is much more random and transient, the molecules detected here formed in the ice coating interstellar dust grains. These grains, of nanometric to micrometric size, serve as substrates on which atoms and simple molecules adsorb. Then they react to each other.
Researchers believe that the formation of COMs follows a sequence. First we find the accumulation of simple molecules (H₂O, CO, CH₃OH) in the icy layers. Then reactions induced by residual thermal energy or by cosmic and ultraviolet rays take place. The interactions between these species then lead to more complex carbon structures.
This so-called chemistry grain surface is known in the Milky Way. But its effectiveness in an environment so poor in metals and exposed to intense radiation remained completely uncertain. The results of the study show that surface reactions can then remain active, even in a constrained context. Which considerably expands the known conditions of organic synthesis in the cosmos.
Will Rocha, co-author, specifies: “ These molecules were previously observed in the gas phase in the LMC, but their detection in solid form confirms their origin on the grains “. Confirmation by infrared absorption spectroscopy makes it possible to distinguish the vibrational signatures specific to each chemical bond, unambiguously validating their nature.
This observation extends the scope of areas favorable to prebiotic chemistry in the universe, even where the necessary chemical elements are scarce and the conditions initially considered unfavorable.
Implications for the origin of life and next scientific steps
Identifying these complex organic molecules in a nearby galaxy does not mean that life exists there. However, it reinforces the idea that the chemical precursors necessary for the emergence of living things can form well before the creation of planets, and in extreme environments.
Among the compounds detected, scientists identified spectroscopic clues consistent with glycolaldehyde. It constitutes a molecule involved in the synthesis of ribose — a fundamental sugar in the formation of RNA. Certainly its presence must still be confirmed. Nevertheless, this hypothesis fuels the debate on the exogenous origin of prebiotic molecules, via interstellar ice grains integrated into protoplanetary disks.
This reinforces the so-called scenarios of molecular panspermia. Some of the building blocks of life on young planets could come from bodies rich in ice, such as comets. These molecules would have survived the formation of stellar systems, then integrated into planetary environments currently being structured.
Marta Sewiło's team now plans to extend its research to other protostars in the Large Magellanic Cloud. But also the Small Magellanic Cloud. The goal: to constitute a representative sample making it possible to evaluate the frequency and diversity of COMs in galaxies with varied characteristics. Sewiło specifies: “ We only have four sources in the Milky Way and only one in the LMC with these signatures. We need to enlarge the sample to draw solid conclusions
“.
This work is part of a larger effort to map chemical complexity on a galactic scale. But above all, determine where and how conditions favorable to life can emerge in the observable universe.
Source: Marta Sewiło et al., “Protostars at Subsolar Metallicity: First Detection of Large Solid-state Complex Organic Molecules in the Large Magellanic Cloud”. 2025 ApJL 992 L30
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