Free Astronomy Magazine May-June 2020

16 MAY-JUNE 2020 ASTROBIOLOGY Since biofluorescence would solely be cor- related with UV fluxes triggered by solar flares and independent of the visible light fluxes from the star, the planet-star contrast could increase significantly in the visible spectrum compared to any non-biofluores- cent state. On the emerged surface of the Earth, the biofluorescence of vegetation produces an effect that can be detected from orbit but is too weak to be recognized from another planetary system. More inter- esting is the biofluorescence produced in shallow ocean floors by corals and organ- isms in symbiosis with them. The fluores- cence efficiency of these aquatic biological structures is much higher than that of sur- face vegetation (some coral species re-emit more than 70% of the incident radiation) and high-resolution observations from orbit can distinguish the signals they emit. How- ever, these are fractions of a percentage point of the total signal emitted by all at- mospheric and surface biosignatures. This does not rule out that, on an Earth-like planet orbiting the habitable zone of a par- ticularly active red dwarf, fluorescent life- forms can proliferate and spread to the point of producing a signal observable from another planetary system. Kaltenegger and O’Malley-James investi- gated this possibility, modeling many sce- narios based essentially on what we know about the fluorescence in coral structures and about the planets of some of the clos- est red dwarfs: Proxima Centauri, Ross 128, TRAPPIST-1, and LHS 1140. The two re- searchers used the emission and absorption properties of fluorescent proteins and pig- ments of common corals to create spectra and color models for Earth-like planets or- biting in these systems. By doing this, they took into account the different characteristics of the candidate planet’s surface, atmospheric absorption, and cloud cover. The most optimistic of the modeled scenarios sug- gests that, in the case of a transparent at- mosphere, active b i o f l uo r e s c en c e could produce a temporary in- crease in visible spectrum bright- ness of two orders of magnitude over the planetary bright- ness during the non-fluorescing state. All this, however, is provided that the entire sur- face is covered by a biosphere characterized by highly-effi- cient fluorescent processes − an ide- alized scenario that is highly unlikely to be observed in reality, at least among exoplan- ets at relatively short dis- tances from our planetary system. Countless variables can negatively impact the chances for S pectral diagram showing the different types of UV rays and the colors visible to us. Aquatic fluo- rescent life reacts to the former by emitting visible light almost in- stantly.