By observing the mid-infrared domain, Darwin will be sensitive to spectral signatures of water vapour, ozone and carbon dioxide. The simultaneous presence of these three compounds in a planet’s atmosphere is considered as a reliable signature of biological activity: “Up until now”, explains Olivier Absil, “researchers haven’t found any non-biological process capable of producing these three gases in large quantities. In a balanced atmosphere, oxygen, which is highly oxidising, recombines directly with carbon or hydrogen. A large quantity of ozone can only be maintained in the presence of water vapour and carbon dioxide if a source continuously produces it. And yet, the only known source up until now to produce oxygen and ozone in large quantities is photosynthesis. Other compounds, such as ammonia, methane or nitrogen oxides, are also difficult to explain without biological activities, but they are less easy to detect.”

In the beginning, astronomers had hoped to be able take an important step with the Darwin mission at the end of 2008: from a possible ESA mission, it was supposed to integrate Cosmic Vision, which defines the space missions that are really on the ESA programme for the 2015-2025 period. But in the end, Darwin was not selected. “It was and wasn’t a surprise”, Olivier Absil confides. “At the moment, Darwin hasn’t yet achieved a sufficient level of maturity. On the one hand, the nulling interferometry essential to Darwin hasn’t yet been qualified for a space flight because it has only been demonstrated in a laboratory. On the other hand, we aren’t yet able to make several satellites evolve in formation, with the accurate positioning of each one in relation to the other to the nearest centimetre… However, ESA and NASA are seriously working on it, since several future space missions will require formation flying. Finally, exoplanetary research is a relatively young subject. There still isn’t a very united science committee, capable of supporting a joint project. Committing to a more modest mission together should allow the “exoplanet” community to better structure itself, to then be able to defend Darwin with a single voice.”