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Our first targets in the search for signs of life are orbiting nearby Mstars, such as the planets in the Proxima Centauri, Ross-128, LHS-1140, andTRAPPIST-1 systems. Future ground-based discoveries, and those from the TESSmission, will provide additional close-by targets. However, young M stars tendto be very active, flaring frequently and causing UV fluxes on the surfaces ofHZ planets to become biologically harmful. Common UV-protection methods used bylife (e.g. living underground, or underwater) would make a biosphere harder todetect. However, photoprotective biofluorescence, 'up-shifting' UV to longer,safer wavelengths, could increase a biosphere's detectability. Here we modelintermittent emission at specific wavelengths in the visible spectrum caused bybiofluorescence as a new temporal biosignature for planets around active Mstars. We use the absorption and emission characteristics of common coralfluorescent pigments and proteins to create model spectra and colours for anEarth-like planet in such a system, accounting for different surface features,atmospheric absorption, and cloud cover. We find that for a cloud-free planetbiofluorescence could induce a temporary change in brightness that issignificantly higher than the reflected flux alone, causing up to twoorders-of-magnitude change in planet-star contrast, compared to anon-fluorescent state, if the surface is fully covered by a highly efficientfluorescent biosphere. Hence, UV-flare induced biofluorescence presentspreviously unexplored possibilities for a new temporal biosignature that couldbe detectable by instruments like those planned for the extremely largetelescope and could reveal hidden biospheres.

Biofluorescent Worlds II. Biological fluorescence induced by stellar UV flares, a new temporal biosignature