Free Astronomy Magazine May-June 2020

12 MAY-JUNE 2020 ASTROBIOLOGY favorable scenarios in order to define the most promising targets in the search for life beyond Earth. Knowing what to look for and where it is most likely to be found will allow us to make the most of both the time and the telescopes that will be assigned to these observations, excluding a priori targets that offer fewer chances of successful discovery. Today, only a handful of known exoplanets are of astrobiological interest. The numbers do increase, and others will be added to this list thanks to the TESS (Transiting Exoplanets Survey Satellite) mission, which has so far identified nearly 1800 exoplanet candidates, of which only about 40 have been officially confirmed. Dozens of exoplanets as big as Earth are expected to be discovered during this mission, with some of them in orbit in the habitable zone of their stars. Other exo- planets will be found directly by ground- based telescopes and added to the growing list. Which of these new worlds will be stud- ied first by the upcoming super-telescopes? Definitely those closest to our Solar System − the easiest ones to separate from their stars. Better still are those nearby systems where the stars are both smaller than the Sun and not very bright, allowing the re- flected light coming from their planets to emerge more distinctly. If we consider the surroundings of the Sun, say up to a distance of 5 parsecs (16.3 light-years), we find about 50 star systems, composed of over 70 stars, of which most are red dwarfs (also called M stars). For reasons related to their formation and evolution, these small stars are very likely to host rocky planets in their habitable zones. It is, therefore, not surprising that most of the potentially habitable planets closest to us are in orbit around red dwarfs. In recent years, astronomers have repeat- edly discussed the usefulness of looking for indicators of possible life (biosignatures) in the light of these planets. G raphical rep- resentation of the structure of the Extremely Large Telescope. When this gigan- tic instrument enters service (perhaps in 2025), it will be the flag- ship telescope in the search for life on other planets. [ESO/H. Zodet]