Trapping in three-planet resonances during gas-driven migration

TL;DR

This study investigates how three-planet resonances form during gas-driven migration and their impact on orbital inclinations, revealing conditions under which systems become three-dimensional with significant mutual inclinations.

Contribution

It provides a detailed analysis of the formation of three-planet resonances and their effects on orbital inclinations during planetary migration, using a simplified analytic model.

Findings

Multiple-planet resonances of 1:2:4 and 1:3:6 are common for planets below 1.5 M_J.

Inclination can grow to ~35 degrees due to resonance and eccentricity pumping.

Trapping in three-planet resonances is rare for certain mass ranges and depends on eccentricity damping.

Abstract

We study the establishment of three-planet resonances -similar to the Laplace resonance in the Galilean satellites- and their effects on the mutual inclinations of the orbital planes of the planets, assuming that the latter undergo migration in a gaseous disc. In particular, we examine the resonance relations that occur, by varying the physical and initial orbital parameters of the planets (mass, initial semi-major axis and eccentricity) as well as the parameters of the migration forces (migration rate and eccentricity damping rate), which are modeled here through a simplified analytic prescription. We find that, in general, for planetary masses below 1.5 M_J, multiple-planet resonances of the form n3:n2:n1=1:2:4 and 1:3:6 are established, as the inner planets, m1 and m2, get trapped in a 1:2 resonance and the outer planet m3 subsequently is captured in a 1:2 or 1:3 resonance with m2. For mild eccentricity damping, the resonance pumps the eccentricities of all planets on a relatively short time-scale, to the point where they enter an inclination-type resonance (as in Libert & Tsiganis 2011); then mutual inclinations can grow to ~35{\deg}, thus forming a "3-D system". On the other hand, we find that trapping of m2 in a 2:3 resonance with m1 occurs very rarely, for the range of masses used here, so only two cases of capture in a respective three-planet resonance were found. Our results suggest that trapping in a three-planet resonance can be common in exoplanetary systems, provided that the planets are not very massive. Inclination pumping could then occur relatively fast, provided that eccentricity damping is not very efficient so that at least one of the inner planets acquires an orbital eccentricity higher than e=0.3.