Reversing type II migration: resonance trapping of a lighter giant protoplanet

TL;DR

This paper investigates how a lighter giant protoplanet can be trapped in resonance with a more massive one, leading to outward migration, which may explain high eccentricities of some exoplanets.

Contribution

It demonstrates that a lighter giant protoplanet can reverse inward migration through resonance trapping, a novel mechanism affecting planet formation theories.

Findings

Resonance trapping can cause outward migration of a lighter giant protoplanet.

The torque imbalance favors outward movement due to differential Lindblad torques.

This mechanism may explain high eccentricities in some exoplanets.

Abstract

We present new results related to the coupled evolution of a two giant planet system embedded in a protoplanetary disk, in which a Saturn mass protoplanet is trapped in an outer mean motion resonance with a Jupiter mass protoplanet. The gaps opened in the disk by the two planets overlap, therefore the two planet system exchanges angular momentum with the disk at the inner planet's Inner Lindblad Resonances (ILRs) and at the outer planet Outer Lindblad Resonances (OLRs). Since the torques are proportional to the square of the planet masses, and since the inner planet is about three times more massive than the outer one, the ILR torques are favored by a factor ~10 with respect to the one planet case. In the case presented here, this leads to a positive differential Lindblad torque and consequently an outwards migration. We briefly discuss the long-term behavior of the system, which could account for the high eccentricities of the extra-solar planets with semi-major axis a>0.2 AU.