Free Astronomy Magazine September-October 2019

SEPTEMBER-OCTOBER 2019 planets destined to permanently orbit in the habitable zone of M dwarfs when the stars reach the main sequence. In fact, even before they reach main sequence sta- tus, all the water possibly present on their orbiting planets would be irremediably evaporated and missing in space. After reaching the main sequence, intense stel- lar flares would complete the planetary sterilization. In the past, we were almost certain that the first biosignatures (molecules that can be related to the metabolic functions of living beings) would be found around an M dwarf. Today, it seems rational to heavily thin out this group of candidate Earths on the eve of the entry onto the scene of new and powerful telescopes pre- cisely designed to directly observe the atmospheres of those planets. The hopes of identifying convinc- ing biosignatures beyond our solar system then move towards G and K dwarfs. Unfortunately, even including some super-Earths at the lower limits of mass and diameter, we have no more than a handful of known planets that could the- oretically offer environments com- parable to terrestrial ones. After they have selected their targets, what will astronomers look for in those planetary atmospheres? their birth, but also later though less fre- quently, M dwarfs show a particularly vio- lent surface activity, characterized by large-scale flares able to flood their habit- able zone with intense X-ray and UV flows. Finally, the small size of the orbits of po- tential Earths hosted by M dwarfs speed up the synchronization between rotation and revolution periods, a process that makes almost all of a planetary surface in- hospitable. These peculiarities entail harm- ful consequences for the habitability of T his is a list of the exoplan- ets that are more likely to have a rocky composition and maintain sur- face liquid water (i.e. 0.5 < Planet Radius ≤ 1.5 Earth radii or 0.1 < Planet Minimum Mass ≤ 5 Earth masses). They are represented artis- tically. On the side, Giada Arney, author of the study mentioned in the article, with the primary mirror of NASA’s James Webb Space Tele- scope. [NASA]