Dynamics of the TWA 7 planetary system and possibility of an additional planet

TL;DR

This study models the TWA 7 system's planetary architecture, suggesting an undetected inner planet influences the debris disk's inner edge and co-orbital structures, with implications for early planetary system evolution.

Contribution

It combines N-body simulations and secular theory to constrain the properties of an unseen inner planet shaping the debris disk and co-orbital material around TWA 7.

Findings

Inner edge of the disk can be explained by a sub-Jovian planet between 13-23 au.

System is dynamically cold with nearly circular, coplanar orbits.

Constraints confine the inner planet to a narrow parameter space.

Abstract

The debris disk surrounding the young star TWA 7 exhibits morphological features that tightly constrain its planetary architecture. JWST/MIRI observations have recently revealed a directly imaged outer planet at large separation. The disk also displays a sharply defined inner edge near 23 au and an extended asymmetric structure that may trace a horseshoe-like distribution of material indicative of gravitational interactions between planets and planetesimals. We investigate whether the observed disk morphology and the possible co-orbital material can be explained by the combined gravitational influence of the known outer planet and an undetected inner companion. We aim to identify planetary configurations consistent with both the disk structure and the long-term stability of the system. We combined N-body simulations and secular perturbation theory to explore how an undetected inner planet could shape the inner edge of the disk while maintaining the dynamical coldness required for stable co-orbital structures around the outer planet. The analytical framework quantifies the secular coupling between the two planets and delineates dynamically viable configurations. The inner edge of the disk near 23 au can be reproduced by a sub-Jovian planet orbiting between 13 and 23 au. Secular interactions further restrict this companion to nearly circular orbits, as higher eccentricities would excite the outer planet and destabilize the co-orbital material. Together, these constraints confine the system to a narrow region of parameter space. The TWA 7 system appears dynamically cold, with all components, including the planets and the debris disk, sharing nearly circular and coplanar orbits. Such a quiescent configuration likely reflects the weak dynamical stirring, making it a promising laboratory to study the early interplay between planet formation, co-orbital dynamics, and debris-disk evolution.