Dynamical Effects on the Habitable Zone for Earth-like Exomoons

Abstract

With the detection of extrasolar moons (exomoons) on the horizon, it is important to consider their potential for habitability. If we consider the circumstellar Habitable Zone (HZ, often described in terms of planet semi-major axis and orbital eccentricity), we can ask, "How does the HZ for an Earth-like exomoon differ from the HZ for an Earth-like exoplanet?" For the first time, we use 1D latitudinal energy balance modelling to address this question. The model places an Earth-like exomoon in orbit around a Jupiter mass planet, which in turn orbits a Sun-like star. The exomoon's surface temperature is evolved under the action of stellar insolation, atmospheric cooling, heat diffusion, eclipses and tidal heating. We use this model to carry out two separate investigations. In the first, four test cases are run to investigate in detail the dependence of the exomoon climate on orbital direction, orbital inclination, and on the frequency of stellar eclipse by the host planet. We find that lunar orbits which are retrograde to the planetary orbit exhibit greater climate variations than prograde orbits, with global mean temperatures around 0.1 K warmer due to the geometry of eclipses. If eclipses become frequent relative to the atmospheric thermal inertia timescale, climate oscillations become extremely small. In the second investigation, we carry out an extensive parameter study, running the model many times to study the habitability of the exomoon in the 4-dimensional space composed of the planet semi-major axis and eccentricity, and the moon semi-major axis and eccentricity. We find that for zero moon eccentricity, frequent eclipses allow the moon to remain habitable in regions of high planet eccentricity, but tidal heating severely constrains habitability in the limit of high moon eccentricity, making the habitable zone a sensitive function of moon semi-major axis.