A Mechanism of Exciting Planetary Inclination and Eccentricity through a Residual Gas Disk

TL;DR

This paper proposes a new mechanism involving residual gas disks that can excite planetary inclinations and eccentricities, potentially explaining observed diverse exoplanet orbits.

Contribution

It introduces a set of evolution equations modeling the disk-planet interactions, revealing conditions under which planetary inclinations and eccentricities can be significantly excited.

Findings

Residual gas disks can induce mutual inclination and eccentricity in planetary systems.

A massive outer planet can trigger Kozai resonance with an inner planet under specific conditions.

The mechanism explains the origin of inclined, eccentric, and retrograde exoplanet orbits.

Abstract

Accordling to the theory of Kozai resonance, the initial mutual inclination between a small body and a massive planet in an outer circular orbit is as high as $\sim39.2^{\circ}$ for pumping the eccentricity of the inner small body. Here we show that, with the presence of a residual gas disk outside two planetary orbits, the inclination can be reduced as low as a few degrees. The presence of disk changes the nodal precession rates and directions of the planet orbits. At the place where the two planets achieve the same nodal processing rate, vertical secular resonance would occur so that mutual inclination of the two planets will be excited, which might trigger the Kozai resonance between the two planets further. However, in order to pump an inner Jupiter-like planet, the conditions required for the disk and the outer planet are relatively strict. We develop a set of evolution equations, which can fit the N-body simulation quite well but be integrated within a much shorter time. By scanning the parameter spaces using the evolution equations, we find that, a massive planet ($10M_J$) at 30AU with $6^o$ inclined to a massive disk ($50M_J$) can finally enter the Kozai resonance with an inner Jupiter around the snowline. And a $20^{\circ}$ inclination of the outer planet is required for flipping the inner one to a retrograde orbit. In multiple planet systems, the mechanism can happen between two nonadjacent planets, or inspire a chain reaction among more than two planets. This mechanism could be the source of the observed giant planets in moderate eccentric and inclined orbits, or hot-Jupiters in close-in, retrograde orbits after tidal damping.