Free Astronomy Magazine May-June 2020

18 MAY-JUNE 2020 ASTROBIOLOGY fluorescence is triggered by the UV radia- tion, the entire process might last a few hours, but the maximum peak (the reaction associated with the peak of the flare) would last only about a quarter of an hour, too short a time to hope to record an event that started with the planet near the con- junctions with the star. Now that it is clear that we will be able to observe only half of the planetary hemi- sphere or a little more, it is necessary that the red dwarf be particularly active in order to have a greater chance of observing bio- fluorescence. Among those examined by the Cornell researchers, only Proxima Cen- tauri and TRAPPIST-1 are considered active enough to guarantee at least some daily flares. Considering that only a small frac- tion of these flares will pour any UV flux on the part of the planetary surface visible from time to time from the Earth and that it is also not foreseeable when this might happen, it will not be easy for researchers to obtain with ELT and Webb a sufficient amount of time-telescope to observe the hypothetical biological signal. Even while we want to be overly optimistic about the possibility of observ- ing biofluorescence from exoplanets, there is an evolutionary characteris- tic of red dwarfs that poses a perhaps insur- mountable challenge. When these little stars go through their slow pre- main sequence phase, which can last up-to about 2 billion years, they are much brighter and warmer than they will be after reaching the main sequence, and this is be- cause they are still con- tracting under their own gravity (a process that heats the core). There- fore, the planets we see today orbiting in the hab- itable zones of Proxima Centauri (almost 5 billion years), Ross 128 (about 10 billion years), TRAPPIST-1 (about 8 billion years), R oss 128 above) and Proxima Centauri (with its orbit around Alpha Centauri, on the left) are two red dwarfs that host poten- tially habitable planets. They could be interest- ing targets to test the hypothesis of Kaltenegger and O’Malley-James. [Sloan Digital Sky Survey, P. Kervella (CNRS/U. of Chile/ Observatoire de Paris/LESIA), ESO/ DSS2, D. De Mar- tin/M. Zamani]