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