From observational constraints to dynamics: reconstructing the orbital architecture of young planetary systems

Thesis defense of Antoine Lacquement - Friday, October 16 at 2.30pm - Salle Manuel Forestini IPAG

This thesis focuses on reconstructing the orbital architecture of planetary systems from observational constraints that are inherently incomplete. Some bodies remain unseen, while the orbits of detected bodies are often observed over only short arcs and remain affected by substantial uncertainties. Circumstellar disks nevertheless provide indirect access to this architecture, as their morphology preserves the imprint of gravitational perturbations and, more broadly, of the dynamical history of the system. The task is therefore to infer an orbital architecture from fragmentary clues. The objective is to identify configurations compatible with these clues and then assess their physical consistency using dynamical and morphological constraints.
The method developed here relies on two complementary steps. A Keplerian orbital fit first explores the full range of solutions compatible with the observations of the bodies in the system. These solutions are then used as initial conditions for dynamical N-body simulations in which the disk is represented by a set of particles. These simulations serve a dual purpose. On the one hand, they rule out configurations that are unstable or incompatible with the observed disk structure. On the other hand, they enable the exploration of new dynamical scenarios that may reveal the presence of unseen bodies.
Applied to three complementary case studies, this approach demonstrates its discriminating power. In Beta Pictoris, the disk morphology cannot be reproduced by the two planets known at the time, whose orbits are well constrained, and requires the presence of an additional planet. The observed disk structure and the stability of the resulting three-planet system then place strong constraints on the mass, semi-major axis, and eccentricity of this planet. The subsequent detection of a planet whose characteristics agree with the resulting predictions supports the proposed scenario. In TWA 7, direct observations only weakly constrain the orbit of the detected planet. The disk morphology therefore provides most of the discriminating power and likewise requires the presence of an additional planet, which remains undetected to date. Preserving the observed fine co-orbital structure also requires very low eccentricities for any body capable of perturbing it. Finally, GG Tauri is a multiple system that provides a demanding test of the method. The still-incomplete orbital constraints and the dynamical complexity of the system make it particularly well suited to assessing the robustness of the method. Many configurations remain compatible with the orbital data for the stars in this system, but only a few reproduce the morphology of the circumtriple disk. Among these, one noteworthy configuration involves a secular resonance between the inner triple system and the disk particles. This thesis thus establishes a general method for reconstructing the orbital architecture of planetary systems from incomplete observational constraints by combining orbital fitting, dynamical simulations, and disk morphology.

Thesis Direction

• Hervé Beust & Gaspard Duchêne

The Jury will consist of:

• Sean Raymond, Senior researcher LAB Bordeaux, Referee
• David Ehrenreich, Associate Professor, U. Genève, Referee
• Nadège Meunier, Astronomer IPAG UGA, Examiner
• Virginie Faramaz, Associate Professor, U. Arizona, Tucson, Examiner
• Nicolas Cuello, Junior Professor, IPAG UGA, Examiner