Every molecule is like a set of building blocks: the same pieces can be assembled in different ways. Two assemblies with the same atoms but different geometry are called isomers. If one can be turned into the other by a simple rotation around a bond, they are conformers, like a bent arm versus a straight one. If you need to break a bond and swap groups, they are geometric isomers, like a left and right glove.
In the interstellar medium, such pairs are not rare. The authors gathered data on 16 pairs: 13 conformers and 3 geometric isomers. The energy difference between the forms ranges from 10 to 2667 kelvins (in temperature units). They were observed in clouds with temperatures from 7.5 to 300 kelvins. The abundance ratio of the "high-energy" to "low-energy" form varies from 0.009 to 4. If heat were the only factor, high-energy forms would be scarce. But in cold clouds, they are often more abundant than expected.
So in space, it's not just heating and cooling at work. The forms may be produced at different rates on dust grain surfaces, change geometry under the influence of light, or during ice evaporation. These processes need to be included in models of interstellar chemistry. This requires laboratory spectra of high-energy isomers and quantum-chemical calculations of transitions between the forms.
The energy difference between two forms of a molecule can be smaller than the energy of a single microwave photon, yet in a cold cloud these forms may not interconvert for millions of years.