Laplace-Legendre expansion of the planar planetary Hamiltonian

TL;DR

This paper introduces a hybrid Laplace-Legendre expansion for planetary Hamiltonians that accurately models long-term planetary dynamics and demonstrates its broad applicability and convergence properties.

Contribution

It develops the first-order secular Hamiltonian using a novel hybrid expansion that combines classical Laplace and Legendre methods, improving modeling of planetary systems.

Findings

The new expansion maintains exact dependence on eccentricity and semi-major axis ratio.

It shows consistent convergence across diverse orbital configurations.

It bridges the applicability gap between classical Laplace and Legendre series.

Abstract

We explore a hybrid expansion of the disturbing function in planetary dynamics that combines elements of the classical Laplace and Legendre developments. This formulation retains the structure of the Laplace expansion, but expresses the inverse of the mutual distance as a series whose terms keep an exact dependence on both the eccentricity and the semi-major axis ratio. We use it to construct the first-order secular Hamiltonian of the planar 3-body problem, relevant for modeling the long-term evolution of planetary systems. We assess the convergence of the new expansion numerically and compare it with that of the Laplace and Legendre series across a range of orbital configurations. The results show that the new expansion provides consistent performance across diverse dynamical regimes, bridging the domains of applicability of the two classical approaches.