  We investigate the influence of lunar-like satellites on the infrared orbital
light curves of Earth-analog extra-solar planets. Such light curves will be
obtained by NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin missions as
a consequence of repeat observations to confirm the companion status of a
putative planet. We use an energy balance model to calculate disk-averaged
infrared (bolometric) fluxes from planet-satellite systems over a full orbital
period (one year). The satellites are assumed to lack an atmosphere, have a low
thermal inertia like that of the Moon and span a range of plausible radii. The
planets are assumed to have thermal and orbital properties that mimic those of
the Earth while their obliquities and orbital longitudes of inferior
conjunction remain free parameters. Even if the gross thermal properties of the
planet can be independently constrained (e.g. via spectroscopy or
visible-wavelength detection of specular glint from a surface ocean) only the
largest (approximately Mars-size) lunar-like satellites can be detected by
light curve data from a TPF-like instrument (i.e. one that achieves a
photometric signal-to-noise of 10-20 at infrared wavelengths). Non-detection of
a lunar-like satellite can obfuscate the interpretation of a given system's
infrared light curve so that it may resemble a single planet with high
obliquity, different orbital longitude of vernal equinox relative to inferior
conjunction and in some cases drastically different thermal characteristics. If
the thermal properties of the planet are not independently established then the
presence of a lunar-like satellite cannot be inferred from infrared data, thus
demonstrating that photometric light curves alone can only be used for
preliminary study of extra-solar Earth-like planets.
