Apsidal Alignment and Anti-Alignment of Planets in Mean-Motion Resonance: Disk-Driven Migration and Eccentricity Driving

TL;DR

This paper investigates how disk-driven migration influences the apsidal alignment or anti-alignment of planets in mean-motion resonance, revealing that eccentricity driving can produce aligned configurations contrary to standard damping effects.

Contribution

It demonstrates that eccentricity driving from protoplanetary disks can lead to apsidal alignment in resonant planets, challenging the traditional view of anti-alignment from eccentricity damping.

Findings

Standard migration leads to apsidal anti-alignment.

Eccentricity driving can produce apsidal alignment.

Resonance configurations can be complex and irregular.

Abstract

Planets migrating in their natal discs can be captured into mean-motion resonance (MMR), in which the planets' periods are related by integer ratios. Recent observations indicate that planets in MMR can be either apsidally aligned or anti-aligned. How these different configurations arise is unclear. In this paper, we study the MMR capture process of migrating planets, focusing on the property of the apsidal angles of the captured planets. We show that the standard picture of MMR capture, in which the planets undergo convergent migration and experience eccentricity damping due to planet-disc interactions, always leads to apsidal anti-alignment of the captured planets. However, when the planets experience eccentricity driving from the disc, apsidally aligned configuration in MMR can be produced. In this configuration, both planets' resonance angles circulate, but a "mixed" resonance angle librates and traps the planets near the nominal resonance location. The MMR capture process in the presence of disc eccentricity driving is generally complex and irregular, and can lead to various outcomes, including apsidal alignment and anti-alignment, as well as the disruption of the resonance. We suggest that the two resonant planets in the K2-19 system, with their moderate eccentricities and aligned apsides, have experienced eccentricity driving from their natal disc in the past.