In the cold cloud TMC-1, where the temperature never rises above ten degrees above absolute zero, molecules rarely encounter one another. But every so often a helium atom whizzes past, and that brief touch is enough to alter the molecule's rotation. Such collisions dictate what radiation we see from Earth.

The cyclopentadiene molecule is like an orchestra, where each rotational level is a musician with their own part. Collisions with helium act as the conductor: they distribute energy among the levels, boosting some and muting others. Without a conductor, the orchestra would quickly fall out of tune, and its sound would no longer reflect the temperature of the hall.

The researchers calculated exactly how helium 'conducts' cyclopentadiene. To do this, they built a map of the interaction between the two particles and traced all possible outcomes of their encounters at different energies. It turned out that the molecule prefers to keep the orientation of its rotation: transitions where one of the rotational axes does not change occur 5–10 times more often than those where everything changes.

When these data were fed into a radiative transfer model for TMC-1 conditions, it turned out that even at a gas density of about a thousand particles per cubic centimeter, the cyclopentadiene levels are almost completely thermalized. This means the molecule 'sings' as if its temperature matched that of the cloud. So astronomers can safely use the simple assumption of equilibrium to estimate how much cyclopentadiene is in the cloud.

The cyclopentadiene orchestra in the cold cloud TMC-1 needs no elaborate tuning: rare touches from helium are enough to keep all the musicians playing in unison with the temperature of the hall. Now astronomers have an accurate score for these touches, and future observations of other ring molecules can be interpreted much more reliably.