Free Astronomy Magazine November-December 2019

13 NOVEMBER-DECEMBER 2019 The wide range of these values keeps open many different scenarios of how K2-18 b might actually be structured, from a rocky planet with an extended atmosphere to something similar to a mini-Neptune. In any case, we are very likely in the presence of an uninhabitable planet. Even if K2-18 b had a solid surface with ex- panses of liquid water (the maximum we can hope for), the planet could have a high enough surface pressure and surface grav- ity to discourage any visitor. A further negative feature of K2-18 b might be the possible synchronization of the rev- olution and rotation periods. This is not a rare event in billion-year-old red dwarf sys- tems. The age of K2-18 is unknown, mean- ing its planets could be at a stage where they always show the same face to the star, subjecting one side to harmful surface and atmospheric conditions and causing planet- wide extremes related to the redistribution of accumulated heat. Angelos Tsiaras is right when he states that: “This is the only planet right now that we know outside the solar system that has the correct temperature to support water, it has an atmosphere, and it has water in it — making this planet the best candidate for habitability that we know right now.” But he is even more right when he states that: “K2-18 b is not ‘Earth 2.0’ as it is signifi- cantly heavier and has a different atmos- pheric composition. However, it brings us closer to answering the fundamental ques- tion: Is the Earth unique?” reached are essentially the same: the transmission spec- trum of K2-18 b reveals a weak but statistically signifi- cant signature of water ab- sorption at 1.4 µ m. By considering the low tem- perature of the planetary atmosphere, it is plausible to conclude that water vapor can condense into droplets and clouds, perhaps even causing precipitation of liquid water in the mid-atmosphere. The positive notes end here, as the collected spectra and a comparison between atmos- pheric models indicate that the gaseous en- velope surrounding K2-18 b is dominated by hydrogen, with a significant presence of he- lium − a highly toxic mixture for terrestrial- like life. Furthermore, we do not know the mass of the atmosphere and, consequently, not even its density and surface pressure. We know that the total mass of K2-18 b is equal to 8-9 Earth masses and that its diam- eter is between 2.3 and 2.7 times that of our planet (29,300-34,400 km) - the result is an average density between 2.5 and 4.5 g/cm 3 . ! O n the right, Angelos Tsiaras, the leader of the other group of researchers involved in the discovery. [UCL Physics & Astronomy]