Free Astronomy Magazine November-December 2019

6 NOVEMBER-DECEMBER 2019 stated: not even the most powerful tele- scopes currently operating in space or on the ground allow for the unequivocal recognition of the signature left by water molecules in the spectra of atmospheres around planets as small as the Earth (a little more, a little less), not even if these planets orbit around very small stars. The minimum threshold in this field of research is currently represented by the largest super-Earths, whose main atmospheric gases leave a sig- nature (barely distinguishable in the best in- struments) in the tiny fraction of stellar light T his illustration is a good example of how the mass media painted the super-Earth K2-18 b after learning of the discovery of water vapor in its atmosphere. Atmospheric gas was soon transformed into large expanses of surface water. [Harvard University] drunk, even if they are sober. They are, in fact, looking for water in the at- mospheres of planets that, despite possessing essential characteristics for the sus- tainability of life as we know it, may still be planets with properties that make life difficult (not to say impossible) to exist. Those planets are the so-called “super-Earths,” to which rather elastic conventions attribute masses between 2 and 10 Earth masses and di- ameters between 2 and 4 Earth diameters. We know very little about super-Earths because we don't have any in our solar system, yet we know they are the most common plan- ets in the galaxy. We know that it is relatively easy to discover them in transit in front of the disk of their stars when these stars are M-type dwarfs. We also know that these little stars hide non-negligible pitfalls, which make them perhaps totally unsuitable for hosting life on their planets. The positive and negative aspects of the search for potentially habitable planets around red dwarfs were sufficiently ad- dressed in the previous issue of this maga- zine. Here, we want to understand why, although any terrestrial-like habitability on M dwarf super-Earths is unlikely, as- tronomers are today particularly involved in the search for water right in the atmos- pheres of these planets. The reason is simply