  To increase the sample size of future atmospheric characterization efforts,
we build on the planetary infrared excess (PIE) technique that has been
proposed as a means to detect and characterize the thermal spectra of
transiting and non-transiting exoplanets using sufficiently broad wavelength
coverage to uniquely constrain the stellar and planetary spectral components
from spatially unresolved observations. We performed simultaneous retrievals of
stellar and planetary spectra for the archetypal planet WASP-43b in its
original configuration and a non-transiting configuration to determine the
efficacy of the PIE technique for characterizing the planet's nightside
atmospheric thermal structure and composition using typical out-of-transit JWST
observations. We found that using PIE with JWST should enable the stellar and
planetary spectra to be disentangled with no degeneracies seen between the two
flux sources, thus allowing robust constraints on the planet's nightside
thermal structure and water abundance to be retrieved. The broad wavelength
coverage achieved by combining spectra from NIRISS, NIRSpec, and MIRI enables
PIE retrievals that are within 10% of the precision attained using traditional
secondary eclipse measurements, although mid-IR observations with MIRI alone
may face up to 3.5 times lower precision on the planet's irradiation
temperature. For non-transiting planets with unconstrained radius priors, we
were able to identify and break the degeneracy between planet radius and
irradiation temperature using data that resolved the peak of both the stellar
and planetary spectra, thus potentially increasing the number of planets
amenable to atmospheric characterization with JWST and other future mission
concepts.
