  Exoplanets orbiting M dwarf stars are a prime target in the search for life
in the Universe. M dwarf stars are active, with powerful flares that could
adversely impact prospects for life, though there are counter-arguments. Here,
we turn flaring to advantage and describe ways in which it can be used to
enhance the detectability of planets, in the absence of transits or a
coronagraph, significantly expanding the accessible discovery and
characterization space. Flares produce brief bursts of intense luminosity,
after which the star dims. Due to the light travel time between the star and
planet, the planet receives the high intensity pulse, which it re-emits through
scattering (a light echo) or intrinsic emission when the star is much fainter,
thereby increasing the planet's detectability. The planet's light echo emission
can potentially be discriminated from that of the host star by means of a time
delay, Doppler shift, spatial shift, and polarization, each of which can
improve the contrast of the planet to the star. Scattered light can reveal the
albedo spectrum of the planet to within a size scale factor, and is likely to
be polarized. Intrinsic emission mechanisms include fluorescent pumping of
multiple molecular hydrogen and neutral oxygen lines by intense LyAlpha and
LyBeta flare emission, recombination radiation of ionized and photodissociated
species, and atmospheric processes such as terrestrial upper atmosphere airglow
and near infrared hydroxyl emission. We discuss the feasibility of detecting
light echoes and find that under favorable circumstances, echo detection is
possible.
