Free Astronomy Magazine May-June 2020

19 MAY-JUNE 2020 ASTROBIOLOGY likely, but it is true that exploiting biofluo- rescence may be the only way to discover life around the red dwarfs, which account for about 75 percent of known stars. We end with the conclusions of Kaltenegger and O’Malley-James, as reported in their ar- ticle Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems: “While a multitude of factors ultimately determine an individ- ual planet’s habitability, our results demon- strate that high UV radiation levels may not be a limiting factor. The compositions of the atmospheres of our nearest habitable exoplanets are currently unknown; how- ever, if the atmospheres of these worlds re- semble the composition of Earth’s atmos- phere through geological time, UV surface radiation would not be a limiting factor to the ability of these planets to host life. Even for planets with eroded or anoxic atmos- pheres orbiting active, flaring M stars the surface UV radiation in our models remains below that of the early Earth for all cases modelled. Therefore, rather than ruling these worlds out in our search for life, they provide an intriguing environment for the search for life and even for searching for al- ternative biosignatures that could exist under high-UV surface conditions.” LHS 1140 (over 5 billion years), and many other red dwarfs have all been subjected to extreme conditions in their first billion years, probably sufficient to make the plan- ets sterile forever. Let’s add that, inevitably, the innermost planets of the aged red dwarfs have rota- tion and orbital periods tidally locked and, therefore, are always presenting the same hemisphere to their star, with easily imagi- nable consequences. That situation makes the prospects for finding any lifeforms un- T he other two red dwarfs taken into consid- eration in the Kaltenegger and O’Malley-James models: LHS 1140 (in the center of the image on the right) and TRAP- PIST-1 (below). [ESO, Sloan Digi- tal Sky Survey] !