Seasonality on terrestrial extrasolar planets: inferring obliquity and surface conditions from infrared light curves

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We investigate the relationship between the seasonality of hypothetical terrestrial extrasolar planets and their infrared light curves. The moderate obliquity of the Earth leads to seasonal variation in insolation and surface temperatures and, consequently, the infrared radiation emitted to space. The orbital parameters of known extrasolar planets and planet formation theory suggest that terrestrial exoplanets may have more eccentric orbits and/or larger obliquities than that of the Earth. These planets will experience larger variations in stellar radiation: we propose that changes in the emitted infrared flux along the orbit might be detectable by a future planet-finding observatory and that orbital infrared light curves contains information about the obliquities and thermal properties of these objects complementary to other data (e.g., spectroscopy). We present an analytical energy balance model that includes both meridional heat transport and the thermal inertia of an ocean/atmosphere. We use this model to calculate infrared light curves for different obliquities, orbital eccentricities, and viewing geometries. We show that: (1) the infrared orbital variability of “blackbody” planets lacking atmospheres or oceans is comparable to the mean value; (2) the shape of the light curve is sensitive to the obliquity of a planet and the location of the equinoxes along the orbit; and (3) the light curve of an Earth-like planet with a global ocean and atmosphere is dramatically attenuated and shifted in phase relative to a blackbody planet. We investigate the distribution of infrared light curve amplitudes in a statistical ensemble of blackbody and Earth-like planets. We examine the question of light-curve uniqueness and conclude that to independently constrain the major parameters it will be necessary to measure the amplitude and phase of higher (suborbital) harmonics in the light curve. Alternatively, measurements of seasonally-varying parameters other than IR flux must be used.