Do instabilities in high-multiplicity systems explain the existence of close-in white dwarf planets?

TL;DR

This study suggests that dynamical instabilities in multi-planet systems around main-sequence stars can naturally lead to the formation of close-in planets orbiting white dwarfs, explaining observed pollution without complex migration scenarios.

Contribution

It introduces a new dynamical instability-based mechanism for the origin of close-in white dwarf planets, using simulations of multi-planet systems evolved to the WD phase.

Findings

Higher multiplicity increases the likelihood of system instability.

Instability peaks within the first Gyr of WD cooling.

Fraction of unstable systems matches observed pollution rates.

Abstract

We investigate the origin of close-in planets and related phenomena orbiting white dwarfs (WDs), which are thought to originate from orbits more distant from the star. We use the planetary architectures of the 75 multiple-planet systems (four, five and six planets) detected orbiting main-sequence stars to build 750 dynamically analogous templates that we evolve to the WD phase. Our exploration of parameter space, although not exhaustive, is guided and restricted by observations and we find that the higher the multiplicity of the planetary system, the more likely it is to have a dynamical instability (losing planets, orbit crossing and scattering), that eventually will send a planet (or small object) through a close periastron passage. Indeed, the fraction of unstable four- to six-planet simulations is comparable to the 25-50$\%$ fraction of WDs having atmospheric pollution. Additionally, the onset of instability in the four- to six-planet configurations peaks in the first Gyr of the WD cooling time, decreasing thereafter. Planetary multiplicity is a natural condition to explain the presence of close-in planets to WDs, without having to invoke the specific architectures of the system or their migration through the von Zeipel-Lidov-Kozai (ZLK) effects from binary companions or their survival through the common envelope phase.