However, this ten-year-old report on the Darwin mission has resulted in a good piece of news, Olivier Absil stresses: “At last, we’ll no longer need the first phase because between now and the launch of the Darwin satellites, other techniques should provide us with a sample of exoplanets similar to our planet. And yet, the first stage of Darwin is also its weak point. Indeed, it’s difficult to include a mission of several million euros in the ESA programme without providing the assurance of being able to detect rocky planets similar to Earth, which are supposed to be its targets for the second phase… even if numerical simulations show that they must gravitate around a large proportion of stars. Observations have also shown that rocky planets equal to 10 terrestrial masses are very common, undoubtedly around 30% of stars. This isn’t equal to the earth’s mass, but it’s likely that the frequency increases when the mass decreases. We are relatively confident that there are a fair number of terrestrial planets hiding in planetary systems not too far away from our solar system and that they could be detected before the launch of the Darwin satellites, which will play in its favour in the future.”

When a catalogue of rocky planets gravitating in the habitable zone of their star has been obtained, there will be a great deal of pressure on astronomers to launch a mission capable of characterising their atmospheres. This will not be possible through simple photometry. From a technological point of view, there are two possibilities: Darwin’s nulling interferometry and the coronagraph of the American TPFC (Terrestrial Planet Finder Coronagraph). A coronagraph is a device designed to block out the light from a star in order to see what is in its orbit.

A fundamental difference between Darwin and TPFC is the wavelength from which the measurements are carried out: TPFC observes in the visible and Darwin in the mid-infrared. Since an instrument’s resolution is better if the wavelength is shorter, it is therefore better with TPFC, which can use a single telescope instead of an interferometer. On the other hand, it is in the mid-infrared that the clearest signatures of biological activity appear.

Finally, it is perhaps the sample of terrestrial exoplanets that will determine which of the two missions will be launched. The one chosen will be the most mature one technologically when the sample is put together… However, TPFC is starting with a handicap in this extraterrestrial race: only the stars – and therefore the planets – which are close to us are accessible with the TPFC coronagraph. Hence, Darwin is still a serious contender… which is simply crying out to grow up…