Addressing the statistical mechanics of planet orbits in the solar system

Abstract

The chaotic nature of planet dynamics in the solar system suggests the relevance of a statistical approach to planetary orbits. In such a statistical description, the time-dependent position and velocity of the planets are replaced by the probability density function (PDF) of their orbital elements. It is quite natural to set up this kind of approach in the framework of statistical mechanics. In the present paper I focus on the collisionless excitation of eccentricities and inclinations by gravitational interactions in a planetary system, the prototype of such a dynamics being the future planet trajectories in the solar system. I thus address the statistical mechanics of the planetary orbits in the solar system and try to reproduce the PDFs numerically constructed by Laskar (2008). I show that the microcanonical ensemble of the Laplace-Lagrange theory accurately reproduce the statistics of the giant planet orbits. To model the inner planets I then investigate the ansatz of equiprobability in the phase space constrained by the secular integrals of motion. The eccentricity and inclination PDFs of Earth and Venus are reproduced with no free parameters. Within the limitations of a stationary model, the predictions also show a reasonable agreement with Mars PDFs and that of Mercury inclination. The eccentricity of Mercury demands in contrast a deeper analysis. I finally revisit Laskar's random walk approach to the time dependence of the inner planet PDFs. Such a statistical theory could be combined with direct numerical simulations of planet trajectories in the context of planet formation, which is likely to be a chaotic process.