Eccentricity modulation of a close-in planet by a companion - application to GJ 436 system

TL;DR

This paper investigates whether a planetary companion could sustain GJ 436b's eccentric orbit through dynamical interactions, concluding it is unlikely that such a companion maintains the eccentricity within the system's age.

Contribution

The study introduces a general method to predict potential locations of companions affecting a planet's orbit and applies it to GJ 436, finding unlikely that a companion maintains its eccentricity.

Findings

Companions in nearby/distance orbits cannot sustain GJ 436b's eccentricity.

GJ 436b's eccentricity likely dampens over time without a companion.

The method predicts possible companion locations but rules out their role in eccentricity maintenance.

Abstract

GJ 436b is a Neptune-size planet with 23.2 Earth masses in an elliptical orbit of period 2.64 days and eccentricity 0.16. With a typical tidal dissipation factor (Q' ~ 10^6) as that of a giant planet with convective envelope, its orbital circularization timescale under internal tidal dissipation is around 1 Gyr, at least two times less than the stellar age (>3 Gyr). A plausible mechanism is that the eccentricity of GJ 436b is modulated by a planetary companion due to their mutual perturbation. Here we investigate this possibility from the dynamical viewpoint. A general method is given to predict the possible locations of the dynamically coupled companions, including in nearby/distance non-resonant or mean motion resonance orbits with the first planet. Applying the method to GJ 436 system, we find it is very unlikely that the eccentricity of GJ 436b is maintained at the present location by a nearby/distance companion through secular perturbation or mean motion resonance. In fact, in all these simulated cases, GJ 436b will undergo eccentricity damp and orbital decay, leaving the present location within the stellar age. However, these results do not rule out the possible existence of planet companions in nearby/distance orbits, although they are not able to maintain the eccentricity of GJ 436b.