Although our solar system features predominantly circular orbits, the exoplanets discovered so far indicate that this is the exception rather than the rule. This could have crucial consequences for exoplanet climates, both because eccentric terrestrial exoplanets could have extreme seasonal variation, and because giant planets on eccentric orbits could excite Milankovitch-like variations of a potentially habitable terrestrial planet,A^os eccentricity, on timescales of thousands-to-millions of years. A particularly interesting implication concerns the fact that the Earth is thought to have gone through at least one globally frozen, snowball state in the last billion years that it presumably exited after several million years of buildup of greenhouse gases when the ice-cover shut off the carbonate-silicate cycle. Water-rich extrasolar terrestrial planets with the capacity to host life might be at risk of falling into similar snowball states. Here we show that if a terrestrial planet has a giant companion on a sufficiently eccentric orbit, it can undergo Milankovitch-like oscillations of eccentricity of great enough magnitude to melt out of a snowball state.
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