Mutual Inclination of Ultra-Short-Period Planets with Time Varying Stellar J2-moment

TL;DR

This paper investigates how the mutual inclinations of ultra-short-period planets evolve over time due to the decreasing stellar J2 moment, revealing implications for their formation history and initial orbital configurations.

Contribution

It provides a detailed analysis of the impact of a time-varying stellar J2 on the mutual inclinations of USPs, offering new constraints on their formation mechanisms.

Findings

Mutual inclination decreases if planets start coplanar with the stellar equator.

Mutual inclination remains nearly unchanged if the outer planet is aligned with the stellar spin.

Final mutual inclinations depend on initial parameters and stellar spin evolution.

Abstract

Systems with ultra-short-period planets (USPs) tend to possess larger mutual inclinations compared to those with planets located farther from their host stars. This could be explained due to precession caused by stellar oblateness at early times when the host star was rapidly spinning. However, stellar oblateness reduces over time due to the decrease in the stellar rotation rate, and this may further shape the planetary mutual inclinations. In this work, we investigate in detail how the final mutual inclination varies under the effect of a decreasing $J_2$. We find that different initial parameters (e.g., the magnitude of $J_2$ and planetary inclinations) will contribute to different final mutual inclinations, providing a constraint on the formation mechanisms of USPs. In general, if the inner planets start in the same plane as the stellar equator (or co-planar while misaligned with the stellar spin-axis), the mutual inclination decreases (or increases then decreases) over time due to the decay of the $J_2$ moment. This is because the inner orbit typically possesses less orbital angular momentum than the outer ones. However, if the outer planet is initially aligned with the stellar spin while the inner one is misaligned, the mutual inclination nearly stays the same. Overall, our results suggest that either the USP planets formed early and acquired significant inclinations (e.g., $\gtrsim30^\circ$ with its companion or $\gtrsim10^\circ$ with its host star spin-axis for Kepler-653c) or they formed late ($\gtrsim$Gyr) when their host stars rotate slower.