Free Astronomy Magazine March-April 2018

43 EXOPLANETS mospheres of most exoplanets discovered so far; indeed, they were mainly searched around stars similar to the Sun because they are considered (for several good reasons) the most likely to host planets like our own. Nonetheless, an in- creasing number of rocky planets have re- cently been discovered around nearby red dwarfs: our readers will surely remember, for example, the fa- mous case of Proxima Centauri b (2016; 5). To ease their discovery, there is not only the favourable S/N ratio but also the fre- quency with which the planets transit. In- deed, the lesse massive the star, the smaller the stable orbits, and therefore the higher the chances of observing transits, including those of planets orbiting in the habitable zone of those dwarf stars. It is estimated that, compared to a solar-type star, plane- tary transits on a red dwarf are 4 to 10 times more frequent. Also, at an equal dis- tance from the star, a planet of mass ‘x’ produces larger varia- tions in the radial ve- locity of a red dwarf than in a star similar to the Sun, and the chance of calculating ‘x’ depends on the magnitude of those variations. When the mass of a planet is known, you may then have a rough idea of the kind of atmo- sphere that sur- rounds it. With the most pow- erful ground-based telescopes, especially those that will become operational in the imminent future, it will finally be possible to accurately characterise that atmosphere. From the above it is clear that if we want, within a few years, to throw the first glimpse into an exoatmosphere potentially conducive to life as we know it, we must necessarily focus on the nearest red dwarfs, discovering new Earth-sized rocky planets and drawing up a list of possible candi- dates for follow-up atmospheric analysis. This is exactly the task entrusted to ExTrA. Although ExTrA is certainly not the first in- strument used in this type of research, it is the first one with optical and electronic so- lutions that allow it to counteract the main limitations afflicting telescopes operating on the ground. As mentioned, the photometric variations produced by a planetary transit in the light curve of a red dwarf are, in the most favourable cases, measurable in fractions of one-hundredth of a magnitude. One can therefore easily imagine how detrimental the movements of air masses in our atmosphere are to measurements. But it’s not just a matter of seeing because when the signal you want to highlight is so small, any inaccuracy in the instruments can produce ‘noise’ greater than the signal sought. It is no coincidence that no ground- based instrument has yet been able to dis- cover exoplanets with diameters less than twice that of the Earth. Even given an instrument that does not in- troduce noise into the measurements and T hree short time-lapse video sequences of the ExTrA in- strument under the La Silla sky. [ESO/Petr Horálek]