Effect of eccentricity on habitability

Habitable Climates: The Influence of Eccentricity
http://arxiv.org/abs/1002.4875

Abstract wrote:

Radiative equilibrium studies that place Earth-like exoplanets on different circular orbits from the parent star do not fully sample the range of plausible habitability conditions in planetary systems. In the outer regions of the habitable zone, the risk of transitioning into a globally frozen "snowball" state poses a threat to the habitability. 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. Since, for constant semimajor axis, the annual mean stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest habitable semimajor axis to scale as (1-e^2)^(-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, 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 long winter at an eccentric planet's apoastron would render it more susceptible to global freezing. Our models suggest that this is not a significant risk for Earth-like planets around Sun-like stars, as considered here, since such planets are buffered by the thermal inertia provided by oceans covering at least 10% of their surface. Nevertheless, the extreme temperature variations achieved on highly eccentric exo-Earths raise questions about the adaptability of life to marginally or transiently habitable conditions.

(emphasis mine)

A second view of this considering whether eccentricity variations in extrasolar planets could push a planet out of a snowball phase...

Generalized Milankovitch Cycles and Longterm Climatic Habitability

Pervasive Orbital Eccentricities Dictate the Habitability of Extrasolar Earths
http://arxiv.org/abs/1003.0633

Abstract wrote:

The long-term habitability of Earth-like planets requires low orbital eccentricities. A secular perturbation from a distant stellar companion is a very important mechanism in exciting planetary eccentricities, as many of the extrasolar planetary systems are associated with stellar companions. Although the orbital evolution of an Earth-like planet in a stellar binary is well understood, the effect of a binary perturbation to a more realistic system containing additional gas giant planets has been very little studied. Here we provide analytic criteria confirmed by a large ensemble of numerical integrations that identify the initial orbital parameters leading to eccentric orbits. We show that an extra-solar earth is likely to experience a broad range of orbital evolution dictated by the location of a gas-giant planet, necessitating more focused studies on the effect of eccentricity on the potential for life.

Hm, I will have to check these out. I'm wondering how quickly a world's climate would respond to high orbital eccentricity (especially if the orbit is close to the star, so the year's shorter and there's less time for the atmosphere to adapt to the extremes).

The first paper quoted in this thread seems to suggest that any significant global water coverage would help to stabilize a planet's climate.

I would expect that if a planet that were close to its star had more severe seasonal variations than a planet farther away, it would be more attributable to being in a steeper temperature gradient than the atmosphere adapting to the changes.

Imagine a pot of water, if you crank the temperature up, then down, then up in rapid succession, the water doesn't have much time to react to the changes in temperature, so the temperature would oscillate less than if you were to walk away and do something else in-between adjusting the temperature. But of course the temperature the planet is does not scale linearly with the distance to the star, so more extreme seasons (I would assume) would be expected for a closer in planet than a farther out planet.

Maybe geological lag (I'm sure there's a real term for that) would help to keep the year roughly stable. Recall that Pluto's atmosphere has gotten denser since it passed through perihelion. Even as Pluto gets farther from the sun, its atmosphere thickens. Maybe for some planets, eccentricity-related effects are damped out by other processes, in addition to the "thermal inertia" provided by ocean cover.