A General Origin for Multi-Planetary Systems With Significantly Misaligned USP Planets

TL;DR

This paper investigates the dynamical evolution of multi-planet systems with ultra-short period planets, focusing on how stellar quadrupole moments influence orbital misalignments over time, using secular theory and N-body simulations.

Contribution

It introduces a framework combining secular perturbation theory and N-body simulations to explain the origin of misaligned USP planets in multi-planet systems.

Findings

Stellar quadrupole moments can cause significant mutual inclinations in USP planets.

The evolution of the TOI-125 system supports the theory of dynamical misalignment origins.

Misalignments can be explained by long-term stellar and planetary interactions.

Abstract

Ultra-short period (USP) planets are exoplanets which have orbital periods of less than one day and are unique because they orbit inside the nominal magnetic truncation gap of their host stars. In some cases, USP planets have also been observed to exhibit unique dynamical parameters such as significant misalignments in inclination angle with respect to nearby planets. In this paper, we explore how the geometry of a multi-planet system hosting a USP planet can be expected to evolve as a star ages. In particular, we explore the relationship between the mutual inclination of the USP planet and the quadrupole moment ($J_2$) of the host star. We use secular perturbation theory to predict the past evolution of the example TOI-125 system, and then confirm the validity of our results using long-term N-body simulations. Through investigating how the misalignment between the candidate USP planet and the three other short-period planets in the TOI-125 system arose, we intend to derive a better understanding of the population of systems with misaligned USP planets and how their observed parameters can be explained in the context of their dynamical histories.