In the cold cloud TMC-1, the temperature is just ten degrees above absolute zero. Molecules there rarely collide with each other, but occasionally a helium atom whizzes past. Such a nudge makes the cyclopentadiene molecule change its rotation and emit light that we detect on Earth.
The molecule behaves like a spinning top that rotates in several directions at once. A collision with helium alters the speed of this rotation. Scientists calculated how often and how strongly helium 'kicks' the top at different temperatures. It turns out that the molecule tends to keep one of its rotation axes unchanged, so some transitions occur 5–10 times more often than others.
When these calculations were applied to the conditions of the TMC-1 cloud, it became clear that even at very low gas density, the molecule quickly reaches thermal equilibrium with its surroundings. This means its rotation matches the cloud's temperature, and astronomers can simply measure the brightness of its emission to determine how much cyclopentadiene is in the cloud. Previously, this was an assumption; now it is backed by precise calculations.
The dipole moment of cyclopentadiene is only 0.419 debye, about 10 times smaller than that of its cyano derivative. Because of this, cyclopentadiene thermalizes at densities around 1000 cm⁻³, whereas the cyano derivative would require a density 100 times higher.