The Effect of Orbital Eccentricity on Planetary Climate and Habitability

Orbital eccentricity causes seasonal changes in global-mean stellar flux. In our Solar System, it helps shape the climates of Mars and Pluto. Exoplanets span a wider range of eccentricities: how does eccentricity affect climate and habitability on worlds around other stars? I show that, on planets with Earth-like oceans, ocean heat capacity and latent-heat storage damp climate variations and keep the climate response tied mainly to orbital-mean stellar flux, even at high eccentricity. Clouds, atmospheric dynamics, and ice further modify this baseline response.

I derive a function for the inner edge of the habitable zone in terms of eccentricity and a nondimensional number: the ratio of a planet’s thermal response time to its orbital period. When the effective heat capacity is sufficiently large, the climate responds slowly and is controlled mainly by the orbital-mean stellar flux. Using a scaling analysis of the energy budget, I show that planets with a mixed-layer depth of 10 m remain close to this limit at nearly all eccentricities. 3D GCM simulations with ExoCAM confirm this baseline scaling for aquaplanets. Higher-order effects modify this picture. At high eccentricity, during periastron summer, convection weakens, and cloud cover drops sharply. This lowers the planetary albedo and warms the climate throughout the orbit without greatly increasing the seasonal temperature cycle. This cloud loss also produces a potentially detectable signal in reflected-light phase curves. Eccentricity also erodes climate bistability between Snowball and warm climate states. As seasonal insolation variations increase, ice self-insulation weakens winter freezing relative to summer melting. The Snowball state therefore disappears at moderate eccentricity (e ~ 0.25). This result adds to growing evidence that Snowball bistability is less robust across planetary parameter space than previously assumed.