Free Astronomy Magazine

EXOPLANETS receives from the Sun; not much, but enough to ensure that any wa- ter on its surface stays liquid, at least in warmer regions and during periods of max- imum insolation. (It must be considered that around noon Ke- pler-186 is as bright in the sky of Kepler- 186f as our Sun appear to us around sunset.) A favourable ground temperature and the resultant presence of liquid water are, how- ever, strongly depen- dent on the existence, composition and thick- ness of the atmo- sphere, of whose pos- sible existence we know nothing. The models of the thermal evolution of planets predict that an object with a diameter less than 1.5 Earth diame- ters has little chances to preserve for long an atmosphere dominated by hydrogen and helium. At Kepler-186f’s distance, these elements would evaporate during the ear- ly stages of a red dwarf‘s existence, since those stars are subject to a very lively sur- face activity and emit intense ultraviolet ra- diations. It is therefore likely that Kepler- 186f retained an atmosphere not too dis- similar to that of the Earth. More difficult, instead, is to make any as- sessment on the mass of that planet, which can only be estimated within a wide values range that has at its extremes a composi- tion exclusively based on water and a com- position of pure iron. In the first case Ke- pler-186f would have a mass equal to 1/3 that of Earth, while in the second case it would weigh almost as much as 4 Earths. If, as it seems more plausible, the planet were to have a composition closer to that of the rocky planets of our solar system (about 2/3 of silicates, 1/3 of iron-nickel, traces of water and other elements), its O f the nearly 1,800 exo- planets known, only a handful orbit in the habit- able zone of their stars and have di- mensions not too dissimilar from those of Earth. This diagram shows them ac- cording to size, distance from the stars and energy received from them (Earth = 1). [Chester Harman, PHL/UPR Arecibo, NASA/JPL] mass should not diverge too much from 1.44 Earth masses. From all this and hypothesize that Kepler- 186f may be a place suitable to host life there is a lot in between, since many other unknown-to-us factors might intervene to radically worsen the scenario. We do not know, for example, if the planet rotates on its axis, or if it always shows the same hemi- sphere to its star, as the other Kepler-186 planets most probably do. Neither we do know if it has a magnetic field capable of protecting a possible biosphere from cos- mic radiations. In fact, we only know that Kepler-186f exists, that it orbits in a poten- tially favourable region and that its size is comparable to that of the Earth. Despite knowing only these few things, its discov- ery is the most important step that could still be taken towards the discovery of a second Earth, and in the coming years, when the instruments available to astron- omers will allow to directly investigate its atmosphere and that of similar planets, we will know if and to what extent those envi- ronments can host life as we know it. n