In the cold cloud TMC-1, where temperatures drop to nine degrees above absolute zero, the QUIJOTE radio telescope picked up a faint 'song' — 22 emission lines from the molecule cyclopent[c,d]indene. This is a flat pattern of eleven carbon atoms, resembling a shard of soot: two hexagons fused with a pentagon, with hydrogen atoms hanging off the edges.
Such molecules are called aromatic — not for their smell, but for the robustness of their carbon framework. They have been found in TMC-1 before, but this one is the rarest of all: it is 2.66 times less abundant than indene and 4.66 times less abundant than phenalene. Scientists believe cyclopent[c,d]indene forms when small hydrocarbon fragments like C2H or C2H3 stick to indene. It's like assembling a mosaic: first small rings, then new pieces grow onto them.
Why does this matter? In dark clouds where stars and planets are born, carbon gradually assembles into complex molecules. This find shows that construction proceeds even in deep cold. Perhaps this is how the first organic molecules arose, from which life later emerged. Now chemists have something to puzzle over: how exactly do such patterns grow from simple fragments in the cosmic cold?
Cyclopent[c,d]indene is a flat piece of graphite less than a nanometer across: if such a sheet were the size of a palm, its thickness would be a hundred thousand times thinner than a human hair.