Let's Sweep: The Effect of Evolving $J_2$ on the Resonant Structure of a Three-Planet System

TL;DR

This paper investigates how the evolving stellar quadrupole moment ($J_2$) influences the secular resonances, orbital eccentricity, and inclination of short-period planets in multi-planet systems, affecting their detectability and stability.

Contribution

It introduces the impact of evolving $J_2$ due to magnetic braking on secular resonances and orbital dynamics of short-period planets in multi-planet systems.

Findings

Evolving $J_2$ shifts the location of secular resonances.

Evolving $J_2$ can excite eccentricity and inclination of short-period planets.

High inclination may hinder transit detection and cause instability.

Abstract

Short and ultra-short planets are a peculiar type of exoplanets with periods as short as a few days or less. Although it is challenging to detect them, already several are observed with many additional candidates. If these planets have formation pathways to their longer period counterparts, they are predicted to reside in multi-planet systems. Thus, gravitational perturbation from potential planetary neighbors may affect their orbital configuration. However, due to their close proximity to their host star, they are also subjects to general relativity precession and torques from the stellar spin quadrupole moment ($J_2$). Here we show that an evolving $J_2$ due to magnetic braking, affects the magnitude and location of secular resonances of the short period planet in a multi-planet system. Thus, driving the short period planet into and out of a secular resonance, exciting the planet's eccentricity and inclination. The high inclination can hinder transit observation, and, in some cases, the high eccentricity may result in an unstable configuration. We propose that evolving $J_2$ in a multi-planet system can be critical in understanding the detectability and stability of short-period planets.