The Equilibrium Temperature of Planets on Eccentric Orbits: Time Scales and Averages

TL;DR

This paper demonstrates that planets with global oceans or thick atmospheres have damped temperature fluctuations on eccentric orbits, and their average temperature can be estimated by orbit-averaged irradiation, which increases with orbital eccentricity.

Contribution

It introduces a method to estimate the equilibrium temperature of eccentric planets considering thermal time scales and orbital averaging effects.

Findings

Temperature fluctuations are damped for planets with deep oceans or thick atmospheres.

Average temperature increases with orbital eccentricity as 1/√(1-e²).

Damping effect is stronger for planets around low-mass stars.

Abstract

From estimates of the near-surface heat capacity of planets it is shown that the thermal time scale is larger than the orbital period in the presence of a global ocean that is well-mixed to a depth of 100 m, or of an atmosphere with a pressure of several tens of bars. As a consequence, the temperature fluctuations of such planets on eccentric orbits are damped. The average temperature should be calculated by taking the temporal mean of the irradiation over an orbit, which increases with $1/\sqrt{1-e^2}$. This conclusion is independent of the orbital distance and valid for Sun-like stars; the damping is even stronger for low-mass main sequence hosts.