Preprint typeset using LATEX style emulateapj v. 03/07/07 HABITABLE CLIMATES: THE INFLUENCE OF ECCENTRICITY

In the outer regions of the habitable zone, the risk of transitioning into a globally frozen “snowball ” state poses a threat to the habitability of planets with the capacity to host water-based life. Here, we use a one-dimensional energy balance climate model (EBM) to examine how obliquity, spin rate, orbital eccentricity, and the fraction of the surface covered by ocean might influence the onset of such a snowball state. For an exoplanet, these parameters may be strikingly different from the values observed for Earth. Since, for constant semimajor axis, the annual mean stellar irradiation scales with (1 − e2)−1/2, one might expect the greatest habitable semimajor axis (for fixed atmospheric composition) to scale as (1 − e2)−1/4. We find that this standard simple ansatz provides a reasonable lower bound on the outer boundary of the habitable zone, but the influence of both obliquity and ocean fraction can be profound in the context of planets on eccentric orbits. For planets with eccentricity 0.5, for instance, our EBM suggests that the greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending on obliquity, with higher obliquity worlds generally more stable against snowball transitions. One might also expect that the lon