Planetary Orbital Equations in Externally-Perturbed Systems: Position and Velocity-Dependent Forces

TL;DR

This paper derives explicit equations of motion for perturbed planetary orbits using orbital elements, considering various perturbations including Galactic tides, and discusses their properties and applications.

Contribution

It introduces a generalized form of Gauss' equations for unaveraged perturbed two-body problems with position and velocity-dependent forces, including specific cases and applications.

Findings

Derived equations for various perturbations including Galactic tides.

Analyzed the breakdown of the adiabatic approximation in certain orbital regimes.

Provided Mathematica code for deriving these equations.

Abstract

The increasing number and variety of extrasolar planets illustrates the importance of characterizing planetary perturbations. Planetary orbits are typically described by physically intuitive orbital elements. Here, we explicitly express the equations of motion of the unaveraged perturbed two-body problem in terms of planetary orbital elements by using a generalized form of Gauss' equations. We consider a varied set of position and velocity-dependent perturbations, and also derive relevant specific cases of the equations: when they are averaged over fast variables (the "adiabatic" approximation), and in the prograde and retrograde planar cases. In each instance, we delineate the properties of the equations. As brief demonstrations of potential applications, we consider the effect of Galactic tides. We measure the effect on the widest-known exoplanet orbit, Sedna-like objects, and distant scattered disk objects, particularly with regard to where the adiabatic approximation breaks down. The Mathematica code which can help derive the equations is freely available upon request.