On the origin of polar planets around single stars

TL;DR

This paper explores how planets with near-polar orbits around single stars may originate from debris discs that remained inclined after a binary star merger, supported by N-body simulations of such systems.

Contribution

It introduces a new hypothesis that planets forming in polar-aligned discs around eccentric binaries can explain observed near-polar planets after binary mergers.

Findings

Polar circumbinary discs remain inclined after mergers.

Binary interactions with gas discs may induce mergers.

Post-merger planets can retain near-polar orbits.

Abstract

The Rossiter-McLaughlin effect measures the misalignment between a planet's orbital plane and its host star's rotation plane. Around 10$\%$ of planets exhibit misalignments in the approximate range $80 - 125^\circ$, with their origin remaining a mystery. On the other hand, large misalignments may be common in eccentric circumbinary systems due to misaligned discs undergoing polar alignment. If the binary subsequently merges, a polar circumbinary disc -- along with any planets that form within it -- may remain inclined near 90$^{\circ}$ to the merged star's rotation. To test this hypothesis, we present $N$-body simulations of the evolution of a polar circumbinary debris disc comprised of test particles around an eccentric binary during a binary merger that is induced by tidal dissipation. After the merger, the disc particles remain on near-polar orbits. Interaction of the binary with the polar-aligned gas disc may be required to bring the binary to the small separations that trigger the merger by tides. Our findings imply that planets forming in discs that are polar-aligned to the orbit of a high-eccentricity binary may, following the merger of the binary, provide a possible origin for the population of near-polar planets around single stars.