Simulations of two-planet systems through all phases of stellar evolution: implications for the instability boundary and white dwarf pollution

Dimitri Veras,Mark C Wyatt,Alexander J Mustill,Amy Bonsor

doi:10.1093/mnras/stt289

Abstract

Exoplanets have been observed at many stages of their host star's life, including the main sequence (MS), subgiant and red giant branch stages. Also, polluted white dwarfs (WDs) likely represent dynamically active systems at late times. Here, we perform 3-body simulations which include realistic post-MS stellar mass loss and span the entire lifetime of exosystems with two massive planets, from the endpoint of formation to several Gyr into the WD phase of the host star. We find that both MS and WD systems experience ejections and star-planet collisions (Lagrange instability) even if the planet-planet separation well-exceeds the analytical orbit-crossing (Hill instability) boundary. Consequently, MS-stable planets do not need to be closely-packed to experience instability during the WD phase. This instability may pollute the WD directly through collisions, or, more likely, indirectly through increased scattering of smaller bodies such as asteroids or comets. Our simulations show that this instability occurs predominately between tens of Myr to a few Gyrs of WD cooling.

Highlights

A planet’s life may be split into four distinct stages: 1) formation and concurrent dynamical excitation, 2) main sequence (MS) evolution, 3) evolution during post-MS stellar phase changes, and 4) white dwarf (WD) evolution
If all 8 simulations for a given semimajor axis ratio are stable over 5 Gyr, we place a blue star at 1010 yr in the appropriate horizontal position, even though the vertical position of the star has no physical meaning and is selected for visual impact
Earth-mass planets fail to go unstable beyond the MS Hill stability limit in all but a few cases; the blue stars continue in an unbroken chain out to a2/a1 ≈ 1.510

Summary

Introduction

A planet’s life may be split into four distinct stages: 1) formation and concurrent dynamical excitation, 2) main sequence (MS) evolution, 3) evolution during post-MS stellar phase changes, and 4) white dwarf (WD) evolution. The star has become a WD, and the planet again enters and remains in a phase of relative dynamical quiescence occasionally punctuated by scattering interactions or external forcing. This general picture, which does not include possibilities such as the capture of free-floating planets, planetary destruction due to supernovae, or multiple host stars, describes the life cycle of the vast majority of known exoplanets. Changes to the dynamics of the planetary system following stellar mass loss has been suggested as a potential cause of increased numbers of planetary bodies scattered onto star-grazing orbits (Debes & Sigurdsson 2002; Jura 2008; Bonsor et al 2011; Debes et al 2012). Even in planetary systems where the planets remain on stable orbits, c 2013 RAS, MNRAS 000, 1–24

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