  In this work, we investigate the properties of young giant planet spectra in
the optical and suggest that future space-based direct imaging missions should
be considering young planets as a valuable and informative science case. While
young planets are dimmer in the optical than in the infrared, they can still be
brighter in the optical than a mature planet of similar mass. Therefore, an
instrument designed to characterize mature planets should also be suitable for
high-precision photometric imaging and spectroscopy of young self-luminous
planets in a wavelength range and at a contrast ratio not attainable from the
ground. We identify known young self-luminous companions which are feasible
targets for WFIRST-CGI and compute spectra for them, including a treatment of
scattering and reflected light at optical wavelengths. Using these results, we
highlight potentially diagnostic spectral features that will be present in the
WFIRST-CGI wavelengths. Expanding to direct imaging missions beyond WFIRST-CGI,
we also use evolutionary models across a grid of masses and planet-star
separations as inputs to compute spectra of hypothetical objects, exploring
when reflected light may contribute to a degree comparable to that of thermal
emission from the residual heat of formation.
