The Effect of Lunar-like Satellites on the Orbital Infrared Light Curves of Earth-analog Planets

Abstract

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.