Up until now, several hundred exoplanets have already been revealed. But the great majority of them are hot giants. They are gaseous planets similar to Jupiter, but on a very tight orbit around their star. Owing to insufficient technology, a rocky planet identical to earth has not already been discovered. And yet, if life exists elsewhere in the universe, we expect to find it on the earth’s sister planet, i.e. on a planet with a mass and radius similar to those of the earth, drifting within the habitable zone of its star. This zone is defined by the distance at which it has to be in relation to the star so that water, if it exists, is in a liquid state. It depends on the type of star: the more it shines, the further the habitable zone is from it.

These researchers are especially interested in liquid water because the majority of biologists believe that it is an essential element for the emergence of life: in order for complex molecules to interact, there needs to be a solvent and liquid water is undoubtedly the most universal, considering the abundance of hydrogen and oxygen in the universe.

Detecting small rocky planets like earth, gravitating relatively close to their star, is a technological challenge because the star’s signal drowns out that of such a host. The Darwin space mission was designed to take up this challenge. It is one of the things to result from the renewal of optical interferometry, initiated by Professor Antoine Labeyrie in the middle of the 1970s. Proposed by a French team in the 1990s, the Darwin mission very quickly caught the attention of the European Space Agency (ESA), which financed it for nearly a decade, in order to develop the necessary technologies.

Initially, Darwin was supposed to have two phases. The first one would focus on the systematic exploration of the habitable zones of 300 relatively close stars over a two-year period. Some thirty exoplanets similar to earth might feature in the nets of this astronomical fishing expedition. The second three-year phase of this search for extraterrestrial life would consist of characterising the atmospheres of these exoplanets, in the hope of finding the signatures of biological activity on some of them… but also of carrying out comparative exoplanetology: the systematic study of planets with slightly different rocky characteristics (volcanism, tectonic plates, mass) will also provide in-depth knowledge of our world and the origins of life that flourished here.