A disk-driven resonance as the origin of high inclinations of close-in planets

TL;DR

This paper proposes a new disk-driven resonance mechanism during planet formation that explains the high inclinations and polar orbits of close-in sub-Neptunes without requiring tidal effects or primordial tilts.

Contribution

It introduces a novel theoretical model involving resonance sweeping and parametric instability triggered by disk dispersal, explaining high inclinations and polar orbits.

Findings

Resonance sweeping excites large planetary inclinations.

General relativistic precession enables polar orbit formation.

Polar sub-Neptunes coexist with low-obliquity Jupiters.

Abstract

The recent characterization of transiting close-in planets has revealed an intriguing population of sub-Neptunes with highly tilted and even polar orbits relative to their host star's equator. Any viable theory for the origin of these close-in, polar planets must explain (1) the observed stellar obliquities, (2) the substantial eccentricities, and (3) the existence of Jovian companions with large mutual inclinations. In this work, we propose a theoretical model that satisfies these requirements without invoking tidal dissipation or large primordial inclinations. Instead, tilting is facilitated by the protoplanetary disk dispersal during the late stage of planet formation, initiating a process of resonance sweeping and parametric instability. This mechanism consists of two steps. First, a nodal secular resonance excites the inclination to large values; then, once the inclination reaches a critical value, a linear eccentric instability is triggered, which detunes the resonance and ends inclination growth. The critical inclination is pushed to high values by general relativistic precession, making polar orbits an inherently post-Newtonian outcome. Our model predicts that polar, close-in sub-Neptunes coexist with cold Jupiters in low stellar obliquity orbits.