Tidal circularization of gaseous planets orbiting white dwarfs

TL;DR

This paper investigates how gaseous planets orbiting white dwarfs undergo tidal circularization, exploring the effects of chaotic f-mode excitation and equilibrium tides on orbital evolution and potential planet destruction.

Contribution

It provides a comprehensive analysis of tidal mechanisms affecting gaseous planets near white dwarfs, highlighting the role of chaotic f-modes and equilibrium tides in orbital and structural evolution.

Findings

Chaotic f-mode excitation occurs only within twice the white dwarf Roche radius.

Ice giants are easily destroyed by internal thermal processes, while gas giants are more resilient.

Tidal evolution constraints can inform observations of planets around young white dwarfs.

Abstract

A gas giant planet which survives the giant branch stages of evolution at a distance of many au and then is subsequently perturbed sufficiently close to a white dwarf will experience orbital shrinkage and circularization due to star-planet tides. The circularization timescale, when combined with a known white dwarf cooling age, can place coupled constraints on the scattering epoch as well as the active tidal mechanisms. Here, we explore this coupling across the entire plausible parameter phase space by computing orbit shrinkage and potential self-disruption due to chaotic f-mode excitation and heating in planets on orbits with eccentricities near unity, followed by weakly dissipative equilibrium tides. We find that chaotic f-mode evolution activates only for orbital pericentres which are within twice the white dwarf Roche radius, and easily restructures or destroys ice giants but not gas giants. This type of internal thermal destruction provides an additional potential source of white dwarf metal pollution. Subsequent tidal evolution for the surviving planets is dominated by non-chaotic equilibrium and dynamical tides which may be well-constrained by observations of giant planets around white dwarfs at early cooling ages.