The Origin of Systems of Tightly Packed Inner Planets with Misaligned, Ultra-Short-Period Companions

TL;DR

This paper investigates how ultra-short period planets become misaligned within tightly packed planetary systems, exploring mechanisms like stellar quadrupole effects and external companions, using the K2-266 system as a case study.

Contribution

It introduces and compares two dynamical mechanisms for misalignment of ultra-short period planets within planetary systems, supported by analysis of the K2-266 system.

Findings

Stellar quadrupole moments can cause inner-outer system decoupling.

External companions with specific orbital parameters can induce misalignment.

Both mechanisms can operate simultaneously, affecting system evolution.

Abstract

Ultra-short period planets provide a window into the inner edge of the parameter space occupied by planetary orbits. In one particularly intriguing class of multi-planet systems, the ultra-short period planet is flanked by short-period companions, and the outer planets occupy a discernibly distinct dynamical state. In the observational database, this phenomenon is represented by a small number of stars hosting systems of tightly packed co-planar planets as well as an ultra-short period planet, whose orbit of is misaligned relative to the mutual plane of the former. In this work, we explore two different mechanisms that can produce an ultra-short period planet that is misaligned with the rest of its compact planetary system: natural decoupling between the inner and outer system via the stellar quadrupole moment, and decoupling forced by an external companion with finely-tuned orbital parameters. These two processes operate with different timescales, and can thus occur simultaneously. In this work, we use the K2-266 system as an illustrative example to elucidate the dynamics of these two processes, and highlight the types of constraints that may arise regarding the dynamical histories of systems hosting ultra-short period planets.