A carbon monoxide molecule has two atoms, so it has almost nothing to do. The carbon and the oxygen can stretch toward and away from each other, or spin around one another. That is the whole repertoire.
The poverty is what makes it a favourite of radio astronomers, says Dmitry Wiebe, an astronomer at the Institute of Astronomy of the Russian Academy of Sciences who studies the chemistry of interstellar matter. Picture a cold cloud at about 10 kelvins. The only energy on offer is the faint warmth of the gas. Carbon monoxide spends it on a handful of energy levels, each one gets a decent share, and every jump between levels shines as a bright line. One line. Another line. A third.
Now give the same cloud a molecule of fifteen atoms. The warmth is the same, but the atoms have a great many more ways to move: bending, twisting, parts of the molecule swinging against other parts. The same small budget is split among all of them. Every motion gets a sliver, so every line is weak. And there are lots of lines, "packed tight," as Wiebe puts it, side by side.
That is why the list of molecules found in space has a ceiling, and the ceiling tells you more about our telescopes than about space. When observations stop at some size, it does not mean bigger molecules are absent. They are most likely there. We just can't pick them out.
The more atoms a molecule has, the more such motions are possible, and the more potential lines it produces. But the energy to excite those motions is always the same.
И чем больше в молекуле атомов, тем больше возможно этих движений, тем больше потенциальных линий порождает эта молекула. Но энергия для возбуждения этих движений всегда одна и та же.
— Dmitry Wiebe, Think Like a Scientist, 15:01
Faintness is only the first problem. Each molecule's lines form a comb, and the cloud holds dozens of molecules. The comb of one lies on top of the comb of another, then a third, then a fiftieth. Add the variants of the same molecule in which one hydrogen has been swapped for its heavier twin, deuterium. Each variant has a comb of its own, shifted a little. The whole thing is a palisade of lines, and the astronomer has to say: this line is that molecule, this one is another. Then they have to show those lines to colleagues and convince them.
There is a further trap. Nobody can look at a jumble of lines and announce a molecule, such as methyl formate. The lines of each candidate must already be on a list, measured in a lab or calculated, and the astronomer checks which ones turn up in the spectrum. So the search covers only what someone thought to look for.
Astronomers have found ways to cope. A study led by Gabi Wenzel, published in Science in 2024, caught 1-cyanopyrene in the cold cloud TMC-1 with the 100-metre Green Bank Telescope. The team did not hunt the faint lines one at a time. They cut a small window of spectrum around each of the 150 brightest expected lines and stacked them into a single averaged line. That line stood out at 14.3 sigma, far above the noise.
The choice of target matters as well. A 2025 review by Izaskun Jiménez-Serra in the Handbook of Astrochemistry counted 81 complex organic molecules newly identified between 2021 and 2025. Almost all came from single-dish radio telescopes aimed at sources with low excitation temperatures. Those sources produce less spectral confusion than compact, hot ones.
A chemist on Earth has it easy by comparison: a flask, a molecule inside, and all the time in the world to examine it. An astronomer outside the Solar System can only look. So when a twenty-atom molecule turns up in the data and survives scrutiny, Wiebe says, "we are wildly happy." Anything larger is probably out there too, in the same crowded band, too faint to pull apart.
