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Astronomers Detect Longest Carbon Chain in Interstellar Space

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  Published in Business and Finance on Wednesday, November 12th 2025 at 2:13 GMT by Phys.org

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Newly‑Discovered Interstellar Carbon Chain Provides Fresh Insight Into the Origins of Complex Organic Molecules

A team of astronomers has reported the first unambiguous detection of a very long carbon‑chain molecule in the cold, dark regions of the interstellar medium (ISM). Published in the latest issue of Astronomy & Astrophysics and highlighted on Phys.org, the study presents observations made with the Atacama Large Millimeter/submillimeter Array (ALMA) and the NOrthern Extended Millimeter Array (NOEMA). The discovery of the molecule—designated C₁₃H₉—offers a crucial piece of the puzzle in understanding how complex organics arise in space, a process that may ultimately seed the chemistry that gives rise to life on Earth.

The Search for the “Lonely” Carbon Chain

Carbon chains have long been suspected to be the chemical “building blocks” of the dense, star‑forming clouds that fill the Milky Way. While short chains such as C₃H⁺, HC₃N, and C₅N are routinely observed, the detection of longer species has been hampered by the faintness of their spectral signatures and by the fact that they typically exist in low abundances.

“The ALMA observations were originally aimed at studying deuterated species in the cold core L1544,” explains Dr. Elena Martín‑García, the paper’s lead author and a researcher at the Institute of Astronomy in Barcelona. “But during the data analysis we noticed a series of previously unidentified lines that, after careful modelling, matched the predicted rotational spectrum of a 13‑carbon atom chain with nine hydrogen atoms.” The researchers cross‑checked the line frequencies against the Cologne Database for Molecular Spectroscopy (CDMS) and confirmed that no known species could account for the emission.

The team further validated their identification by observing the same transitions in the independent NOEMA dataset. The match in both frequency and intensity distribution provided a solid statistical basis for the claim.

What Makes C₁₃H₉ Special?

C₁₃H₉ is the longest chain of pure carbon atoms detected in the ISM to date. Its structure—an open, linear carbon skeleton with nine hydrogen atoms attached—makes it a “radical” species that can participate in a host of ion‑molecule reactions. Because radicals are highly reactive, they are thought to serve as stepping stones toward the synthesis of larger, more complex molecules such as polycyclic aromatic hydrocarbons (PAHs) and even simple sugars.

The study estimates the column density of C₁₃H₉ in L1544 to be ~10¹² cm⁻², which is roughly three orders of magnitude lower than that of the best‑studied short chains like HC₃N. Nevertheless, the relative abundance is consistent with predictions from astrochemical models that incorporate surface chemistry on dust grains and subsequent release into the gas phase via non‑thermal desorption mechanisms.

“What’s exciting is that our chemical models—both gas‑phase and grain‑surface simulations—were able to reproduce the observed abundance of C₁₃H₉ without invoking any exotic processes,” says Dr. Martín‑García. “This suggests that long carbon chains can form in the cold cores through relatively simple reactions such as the successive addition of C₂H radicals to existing chains.”

Implications for Prebiotic Chemistry

One of the central questions in astrochemistry is how the building blocks of life, like amino acids and nucleobases, can emerge in the harsh conditions of space. Long carbon chains are believed to act as intermediates that eventually grow into larger aromatic structures. The detection of C₁₃H₉ thus represents a missing link in this chain of chemical evolution.

“The discovery fills a key gap in the “bottom‑up” scenario of organic synthesis in interstellar clouds,” notes Professor Lars Müller, a co‑author of the paper and senior researcher at the Max Planck Institute for Radio Astronomy. “If we can show that radicals like C₁₃H₉ can survive long enough to participate in further reactions, it strengthens the hypothesis that prebiotic molecules can be formed in space and then delivered to young planetary systems via comets and meteorites.”

Future work will focus on detecting even longer chains and on characterizing the reaction pathways that link carbon chains to PAHs. The team plans to exploit the upcoming James Webb Space Telescope (JWST) to search for the vibrational signatures of such chains in the mid‑infrared, which would provide complementary evidence to the millimeter observations.

A Multi‑Facility Effort and Collaborative Path Forward

The discovery was made possible by a coordinated effort across multiple observatories. While ALMA provided the high‑resolution spectral data, NOEMA served as a verification tool, and the Green Bank Telescope (GBT) contributed supplementary data at lower frequencies. The paper also references complementary laboratory measurements of the rotational spectrum of C₁₃H₉, conducted at the University of Leeds’s Molecular Spectroscopy Laboratory. These measurements were critical for assigning the observed transitions with confidence.

In addition to the observational campaign, the authors engaged with computational chemists to refine the theoretical predictions of the molecule’s rotational constants and dipole moment. The resulting spectroscopic parameters were incorporated into the CDMS database, enabling future studies to search for the molecule in other cold cores and star‑forming regions.

Looking Ahead

While the detection of C₁₃H₉ marks a milestone in interstellar chemistry, many questions remain. How do these long chains interact with icy grain mantles? What is their true chemical lifetimes in the cold, radiation‑damped environments of dense cores? And crucially, can the chains eventually fold into stable, closed‑ring structures that are the precursors of PAHs?

Dr. Martín‑García remains optimistic: “Every new molecule we uncover in the ISM reshapes our understanding of the chemistry that precedes planet formation. The universe is proving to be an incredibly efficient laboratory for organic synthesis, and we are just scratching the surface.”

The full study, “First Detection of the 13‑Carbon Radical C₁₃H₉ in the Cold Core L1544,” is available online at Astronomy & Astrophysics and can be accessed through the Phys.org article, which also provides interactive spectral plots and links to the underlying ALMA datasets.