Discs and architecture of planetary systems

Keywords

Orbital characterisation, debris discs, planet-disc interaction, exozodiacal cloud, dust

Motivations

The ingredients of extra-solar systems are diverse and not limited to the telluric, gaseous and icy planets, observed in our solar system in the same orbital plane at increasing distances from the Sun. We also observe belts of planetesimals or interplanetary dust, at various distance from the central star, as well as planets whose position or orbit inclination are signposts for migrations or dynamic interactions. The study of the architecture of systems, the dynamics of the planets, their links with the planetesimal belts, and the properties of the dust present in these discs can help us to go back to the history of the systems to understand their formation and evolution, in all their diversity, to put our own solar system into perspective.
The study of extra-solar systems has indeed revealed a great diversity of architectures, with analogues of our Kuiper belt that are much more massive or extended, often presenting asymmetric morphologies, planets on sometimes eccentric or misaligned orbits, gas giants or telluric planets on very short orbits, to mention only a few examples. Many scientific questions animate the community on these issues, and our team in particular, such as

  • How can we explain the diversity of observed architectures, both in debris discs and orbital configurations ? What is the link with the properties of the host star, in particular the spectral type?
  • What is the link between the inner regions of planetary systems, sometimes containing super-Earths or hot Jupiters, and the outer regions where gas or ice giants and cooler debris rings reside?
  • What are the properties of the dust, both in the inner regions (zodiacal dust) and at a greater distance from their star (exo-cometarian dust) to trace the conditions of planetesimal assembly and dust growth and their composition.

Objectives

The objective is to explore the architecture of extra-solar systems in all their diversity to understand how planets and their discs have evolved to the configuration observed in class III systems around main sequence stars. This includes

  • the characterisation of planetary orbits to get a global view of the architecture of the systems (Fig. 1)
  • the study of the properties of debris discs or exozodiacal clouds, using the observation of their thermal or scattered emission (Fig. 2) and (Fig. 3)
  • modelling the dynamics of the systems (Fig. 2) and the interactions between planets or between planets and the disc (Fig. 4)

Methods

Obtaining an overall view of an extrasolar system requires a combination of detection techniques, including radial velocities and transits sensitive to shorter-period planets, absolute or relative astrometry made possible by the Gaia mission or interferometry, and direct imaging for the most distant regions around young systems. Our team is developing tools to combine these techniques and constrain the presence of planets. This is one of the objectives of the COBREX ERC grant, of which our team is a member. The analysis of the astrometry of planets requires tools for orbital evolution codes (symplectic integrators, N-body codes, see Fig.4) that also allow the testing of orbital stability. Finally, for the study of dust discs and properties, we rely on radiative transfer tools (MCFOST, GRaTeR), forward modelling and high contrast image processing algorithms adapted to extract the morphology and scattering properties of particles. The ERC grant Dust2Planets and the ANR grant DDISK are in progress on these topics. The interpretation of the signals is facilitated by numerical codes and laboratory experiments (microwave or laser scattering analogies). The presence of zodiacal dust is studied by interferometric techniques and the transport mechanisms of dust towards the inner regions are modelled by numerical codes.

Contacts

Jean-Charles Augereau, Carine Babusiaux, Hervé Beust, Alexis Carlotti, Celia Desgrange, Julien Milli, Arthur Péronne, Sophia Stasevic

Illustrations

Fig. 1: Comparaison des limites de détection obtenues en combinant vitesses radiales avec HARPS (RV) et imagerie directe avec SPHERE (DI) sur 2 systèmes: une étoile jeune à 100 pc , HD95086, hôte d’une planète géante de 5 masses de Jupiter, et GJ 229, naine M de plusieurs milliards d’années à 6 pc, orbitée par une naine brune et 2 super-Terres // Comparison of the detection limit obtained by combining radial velocities with HARPS (RV) with direct imagine with SPHERE (DI) on 2 systems: a young star at 100pc, HD95086, host of a 5-Jupiter-mass giant planet, and GJ 229, red dwarf of several billion years at 6 pc, orbited by a brown dwarf and 2 super-Earths.



Fig. 2: Images multi-époques du disque de débris de AU Mic, montrant des concentrations de poussières en dessus du plan médian du disque se déplacant vers l’extérieur du système // Multi-epoch images of the debris disc around AU Mic, showing concentrations of dust above and below the disc midplane moving outwards at high velocity.



Fig. 3: Le disque de débris HR4796, et ses asymétries de brillances résultant d’une fonction de phase anisotrope des particules de poussières, avec une forte diffusion vers l’avant et une rétrodiffusion. // The debris disc around HR 4796, with brightness asymmetries resulting from the anisotropic phase function of its dust particles with a enhanced forward-scattering and mild backward-scattering.



Fig. 4: Modélisation N corps du disque de planétésimaux entourant l’étoile beta Pictoris, sous l’influence des perturbations gravitationnelles des 2 planètes géantes b et c détectées par imagerie directe et interférométrie / astrométrie / vitesses radiales respectivement // N-body simulation of the planetesimal disc surrounding the star beta Pictoris, under the gravitational influence of 2 giant planets, b and c, detected in direct imaging and radial velocity / astrometry / interferometry respectively.