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Although the Earth's orbit is never far from circular, terrestrial planetsaround other stars might experience substantial changes in eccentricity thatcould lead to climate changes, including possible "phase transitions" such asthe snowball transition (or its opposite). There is evidence that Earth hasgone through at least one globally frozen, "snowball" state in the last billionyears, which it is thought to have exited after several million years becauseglobal ice-cover shut off the carbonate-silicate cycle, thereby allowinggreenhouse gases to build up to sufficient concentration to melt the ice. Dueto the positive feedback caused by the high albedo of snow and ice,susceptibility to falling into snowball states might be a generic feature ofwater-rich planets with the capacity to host life. This paper has two mainthrusts. First, we revisit one-dimensional energy balance climate models astools for probing possible climates of exoplanets, investigate the dimensionalscaling of such models, and introduce a simple algorithm to treat the meltingof the ice layer on a globally-frozen planet. We show that if a terrestrialplanet undergoes Milankovitch-like oscillations of eccentricity that are ofgreat enough magnitude, it could melt out of a snowball state. Second, weexamine the kinds of variations of eccentricity that a terrestrial planet mightexperience due to the gravitational influence of a giant companion. We showthat a giant planet on a sufficiently eccentric orbit can excite extremeeccentricity oscillations in the orbit of a habitable terrestrial planet. Moregenerally, these two results demonstrate that the longterm habitability (andastronomical observables) of a terrestrial planet can depend on the detailedarchitecture of the planetary system in which it resides.

Generalized Milankovitch Cycles and Longterm Climatic Habitability