ANR-DFG - Appel à projets générique 2018 - DFG

Growth and Evolution of Planets in protoplAnetaRy Disks – GEPARD

GEPARD: Growth and Evolution of PlAnets in protoplanetaRy Disks

Planets, in particular giant planets, form in disks of gas and dust around young stars. How they reach their final mass and orbit remains poorly understood. The phenomenons of planetary migration and gas accretion, which are intricated, can only be understood with a new paradigm of protoplanetary disks.

A better disk model for a better understanding of planetary formation

Proto-planetary disks spread and are accreted by the star, and eventually vanish in a few million years. This observation has long been attributed to the effect of a mysterious viscosity, itself often attribued to turbulence caused by the magneto rotationnal instability. But recent results challenge this paradigm. Our goal is first to establish whether protoplanetary disks could be rather dominated by active outer layers, dragged by disk winds, while the bulk of the disk remains laminar, with a minimal viscosity set by the vertical shear instability.<br />Once this structure well established through numerical simulations in magneto-hydrodynamics, we will study the behaviour of giant planets in these disks. How do they interact with the active layers ? Do tey efficiently repel the midplanet layer ? What is the new structure of the gap inevitably open by a giant planet around its orbit, spliting the disk in two , We hope to find that the planets migrate slower than in a viscous disk, because giant planets migration generally happens at a speed proportionnal to the viscosity. This way, we would be able to explain the presence of many giant exoplanets located a several astronomical units from theit host star, contrary to the famous hot Jupiters, situated much closer, who are finally rare. Furthermore, if the gas and the planet are less in contact, this could expain that a giant planet does not become a super giant by runaway accretion of gas. We would then explain a whole part of the distribution of exoplanets in the mass - distance diagram, which is so difficult to reproduce in planet population synthesis models.

We perform numercial simulations using two different grid codes. The grid covers a large region of the protoplanetary disk around the planet, in 2D or 3D. We use spherical coordinates centred on the star, whose equatorial plane in the midplane of the disk. In 2D, we can either neglect the thickness of the disk and simulate the midplane (convenient for planet-disk interactions studies), or assume the disk is axisymmetric and simulate a radial slice, perpendicular to the midplane (convenient for disk structure studies).

Our two codes are FARGOCA (developed at OCA for several years, based on FARGO by F. Masset, dedicated to protoplanetary disks) and PLUTO (a versatile community code). Adaptations of the codes are necessary for the new disk conditions we study. In particular, even before having obtained robust disk structure results, we have implemented a simplified model of this structure to run preliminary simulations.

From october 2019 to end of 2020, in Nice we worked on two aspects of planetary migration while our german partner of hte university of Tübingen focussed on the study of the disk structure. We have revisited the paradigm of type II migration, using 2D numerical simulations to study in detail the torque felt by the planet inside its gap, as a function of the its position inside the gap. The result will be a genzric formula for the migration speed, whose form and physical ground has already been established, and the coefficients remain to be measured and the validity tested. We have also studied the case of giant planets in very low viscosity disks, and found two new migration regimes, depending on the eccentricity of the planet. These results show that, as expected, the migration of giant planets in three dimensional low viscosity disks, can be very slow.

In the almost two years remaining, we will finish the ongoing work, and put together our results with those found by our Tübingen partners, in order to get a global view on the problem.

2 papers submitted to Astronomy & Astrophysics (30/12/2020).

As of today over 2600 exoplanetary systems that contain over 3500 planets have been discovered. The debiased observations show that the most abundant planets are Super-Earths (planets with 1-20 Earth masses) with orbital periods shorter than 100 days, followed by giant planets at distances of 1-3 astronomical units (AU) from the parent star. The latter outnumber, by at least a factor of ten, the population of hot-Jupiters (at a distance of ~ 0.1 AU from the star). The mass distribution of giant planets peaks at about 1-3 Jupiter masses; planets with masses larger than that exist but are quite rare. From a theoretical standpoint, these observations are difficult to understand. Planet migration towards the star can easily explain the existence of close-in super-Earths, but it is a problem to understand why only a minority of giant planets reached orbits less than 1 AU in semi major axis. Also, gas accretion onto planetary cores should be very fast. Thus, it is not understood what prevented super-Earths from becoming giant planets and what limited the growth of giant planets to a few Jupiter masses.

This proposal is based on the idea that the difficulties in understanding the extrasolar planets’ mass and orbital distributions are due to incorrect assumptions on the protoplanetary disk structure. The classic view of a viscous disk, with viscosity generated by strong turbulence driven by the magneto-rotational instability, is challenged by modern magneto-hydrodynamic simulations. Disks are probably much less viscous than previously thought. Nevertheless, disks cannot be inviscid, a minimum viscosity being set by the so-called vertical shear instability (VSI). In addition, disk winds remove angular momentum from thin surface layers of the protoplanetary disk, promoting the fast radial transport of gas towards the central star in these layers. Our proposed project is (i) to construct a realistic model of protoplanetary disks accounting for both the VSI and disk winds, and reproducing the observed stellar accretion rates and (ii) to study the accretion of gas and the migration of planets embedded in these disks.

Project coordination

Aurelien Crida (Laboratoire J.L. LAGRANGE (OCA/CNRS/UNS))

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


EKUT Universitaet Tuebingen
Université Cote d'Azur Université Cote d'Azur

Help of the ANR 452,312 euros
Beginning and duration of the scientific project: October 2018 - 36 Months

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