The interstellar medium and combustion systems contain polycyclic aromatic hydrocarbons (PAHs) as fundamental molecular building blocks that form fullerenes and carbonaceous nanostructures. However, researchers have yet to investigate and understand aromatic molecules carrying five-membered rings that form the essential building blocks of nonplanar polycyclic aromatic hydrocarbons (PAHs), which eventually lead to the formation of interstellar grains or carbonaceous cosmic dust. In a new report now published in Science Advances, Srinivas Doddipatla and a team of scientists in chemistry, physics and astronomy in the U.S. and Russia explored the concept with crossed molecular beam experiments, electronic structure calculations and astrochemical modeling. The work revealed an unusual pathway to form indene (C9H8)—a prototype aromatic molecule with a five membered ring. The mechanism was based on a barrierless biomolecular reaction that involved the simplest organic radical – methylidyne (CH) and styrene (C6H5C2H3) via a hitherto elusive methylidyne addition-cyclization-aromatization (MACA) mechanism. The work offers a new concept on the low-temperature chemistry of carbon found in the galaxy.