Dense interstellar clouds harbor a rich and exotic chemistry. Many interstellar molecules have been identified in such clouds, from small and stable species like carbon monoxide to reactive radicals and long carbon chains such as C8H or HC11N. Despite the extremely low temperatures (10 K above zero) and densities (at most a million particules per cubic centimetre), chemical reactions both in gas and solid phase, enable the synthesis of the known interstellar molecules. It was considered up to now that most interstellar species in dark clouds were either reactive or unsaturated species, in accordance with the detected molecular species. The presence of saturated molecules was expected in the solid phase, but not in the gas phase as the gas phase chemical networks led mostly to the production of reactive and unsaturated species.
The detection of propylene is challenging this hypothesis as this organic molecules reaches a high abundance in TMC-1, comparable to the abundance of molecules of similar size and molecular weight (CH3CCH, c-C3H2). The detection has been more difficult because of the lower dipole moment of the propylene molecule, leading to weaker rotational lines compared to radicals with high dipole moments. The excellent sensitivity and good angular resolution offered by the IRAM 30m telescope have permitted to overcome this observational bias in the exploration of the interstellar medium chemistry. The presence of a relatively simple molecule like propylene demonstrates that the chemical inventory is far from complete and that new discoveries are awaiting us. Systematic surveys of dark clouds, like the on-going work of the team led by J. Cernicharo are necessary to continue the exploration and obtain a more accurate description of the chemistry.
Furthermore, this discovery will lead to significant improvement of the chemical networks used for modeling the interstellar medium. It is expected that the respective roles of gas phase and solid phase chemistry in forming saturated species like propylene will be better evaluated.