How giant planets form is one of the major open issues in planetary science. This limits our understanding of the origin of planetary system architectures and the conditions needed for the formation of habitable terrestrial planets. In this project we will test new ideas on the growth of giant planet cores and planetary embryos by pebble accretion, the interplay between growth and migration, as well as gas accretion by giant planets
The Solar System is not a typical planetary system. Most planetary systems harbour systems of super-Earth planets on short period orbits. Our Solar System does not have these planets. Most planetary systems with known giant planets have thse planets on eccentric and short-period orbit. Our Solar system have giant planets on distant, almost circular orbits. By understanding the early phases of the growth of planets in gaseous proto-planetary disks and the migration of these planets, we aim at understanding the possible structures of planetary systems, their diversity and the place of our Solar System in the general context.
Our research is theoretical and it is conducted via numerical simulations. We use hydro-dynamical codes to study the evolution of the proto-planetary disk, the migration of the planets or the accretion of gas by the planets. We use N-body codes for the subsequent phases, in which the planets evolve after the disappearence of the gas.
From our works, a coherent vision is emerging on the formation of the Solar System in the context of the diversity observed in extra-solar planetary systems. We find  that pebble accretion should produce in general a set of low-mass planetary embryos in the warm part of the disk (i.e. within the snowline) and multi-Earth-mass cores in the cold part of the disk (i.e. beyond the snowline). The migration of the cores then takes place and it generates a variety of possible evolutions, leading eventually to very diverse final planetary systems. The inward migration of multi-Earth-mass cores is initially stopped near the snowline by the presence of a localized outward migration region [2,3,4]. However, with the cooling of the disk the outward migration region disappears and the super-Earths penetrate into the very inner part of the disk, becoming the close-in super-Earths so frequently observed around most stars. The evolution deviates from this general pattern if, by any chance, the innermost core manages to trigger runaway gas accretion and become a giant planet . In this case, the giant planet –which migrates more slowly than the cores- acts as an effective dynamical barrier and prevents the cores to jump across its own orbit . In the Solar System evolution, an additional event occurred with the growth of the second core by gas accretion until reaching the mass of Saturn. The co-presence of Jupiter and Saturn triggered the outward migration of these two planets (Masset and Snellgrove, 2001 and ). This process could have favored the formation of Uranus and Neptune, through a series of giant impact among sub-cores, thus explaining the large obliquities of their spin axes . Thus, in the framework of a coherent model of accretion/migration and its possible evolutionary branches, we now start to be able to understand the diversity of planetary systems, which systems are the most generic and the origin of the peculiar structure of our own system.
In the remaining time for this project, we will try to put the global view of the formation of planetary systems, described above, into the form of a quantitative model. We will focus in particular on (1) the accretion of gas by super-Earths and giant planet cores, (2) the migration of accreting giant planet, in the framewrok of layered disks and (3) the effect of the mass-flux of pebbles into the inner part of a protoplanetary disk, leading to the formation of super Earths or. standard terrestrial planets.
 Morbidelli, A., Lambrechts, M., Jacobson, S., Bitsch, B. 2015. The great dichotomy of the Solar System: small terrestrial embryos and massive giant planet cores, Icarus, 258, 418-429.  Cossou, C., Raymond, S.N., Hersant, F., Pierens, A. 2014. Hot super-Earths and giant planet cores from different migration histories. Astronomy and Astrophysics 569, A56.  Bitsch, B., Morbidelli, A., Lega, E., Crida, A. 2014a. Stellar irradiated discs and implications on migration of embedded planets. II. Accreting-discs. Astronomy and Astrophysics 564, AA135.  Bitsch, B., Johansen, A., Lambrechts, M., Morbidelli, A. 2015. The structure of protoplanetary discs around evolving young stars. Astronomy and Astrophysics 575, AA28.  Lambrechts, M., Johansen, A., Morbidelli, A. 2014. Separating gas-giant and ice-giant planets by halting pebble accretion. Astronomy and Astrophysics 572, AA35.  Izidoro, A., Raymond, S.N., Morbidelli, A., Hersant, F., Pierens, A. 2015a. Gas Giant Planets as Dynamical Barriers to Inward-Migrating Super-Earths. The Astrophysical Journal 800, LL22.  Pierens, A., Raymond, S.N., Nesvorny, D., Morbidelli, A. 2014. Outward Migration of Jupiter and Saturn in 3:2 or 2:1 Resonance in Radiative Disks: Implications for the Grand Tack and Nice models. The Astrophysical Journal 795, LL11.  A. Izidoro, A. Morbidelli, S.N. Raymond, F. Hersant, and A. Pierens. 2015b. Accretion of Uranus and Neptune from inward-migrating planetary embryos blocked by Jupiter and Saturn, A&A 582, A99.
How giant planets form is one of the major open issues in planetary science. Given that gas giants gravitationally dominate their planetary systems, this limits our understanding of the origin of planetary system architectures and the conditions needed for the formation of habitable terrestrial planets.
The standard model of giant planet accretion (Pollack et al 1996) consists of two phases: the accretion of a solid rocky/icy core of several Earth masses and the formation of a massive atmosphere around such a core by the capture of gas from the circumstellar disk. However, each of these phases presents significant unsolved problems. First, the accretion of cores from a population of planetesimals seems to stall long before a mass of several Earth masses is reached. Second, simulations predict that the capture of gas by a massive core, once it enters a runaway phase, does not stop until several Jupiter masses of gas are accreted (unless some fine-tuning of the removal of the circumstellar gaseous disks is invoked), in clear conflict with the masses of Jupiter and Saturn and the population of giant extra-solar planets (in which Jovian and sub-Jovian planets are very abundant).
With MOJO, we propose to carefully study these phases by combining comprehensive hydrodynamical simulations and N-body simulations of interacting planetary embryos embedded in circumstellar disks. We will test two promising new mechanisms for core accretion: (i) rapid formation of planetary embryos via the accretion of pebbles by the largest planetesimals and (ii) mutual accretion of the embryos due to convergent migration. The latter requires a realistic model of non-isothermal protoplanetary disks in which planetary embryos migrate via tidal interactions with the gas towards an equilibrium orbital radius. This equilibrium radius could be a sweet spot for the mutual accretion of embryos, leading to the formation of a few massive cores. Once embryos are massive enough to become giant planets, accretion of their envelope will be studied with a nested mesh code that can resolve the accretion flow and the circumplanetary disk. Recent results show that this disk should be much less ionized and viscous than the surface layers of the circumstellar disk. A low-viscosity circumplanetary disk may act as a bottleneck for gas accretion onto the planet, preventing the fast runaway phase. This process has the potential of increasing the gas accretion timescale, making it comparable with the disk lifetime. The diversity of giant planet masses may stem naturally from the similarities of these two timescales.
Experience has taught us that the Solar System, with its numerous and precise observational constraints, is a key benchmark for formation models. Requiring exoplanet-oriented models to also match the Solar System has led to the identification of essential physical mechanisms that would have otherwise been missed. MOJO will thus focus first on giant planet formation in our Solar System. The study will then be applied to exoplanetary systems, for which constraints on both the giant planet and terrestrial planet populations are becoming more and more detailed thanks to numerous ground-based and space-based surveys. We will also study the consequences of the migration of the planetary embryos on the formation and survival of habitable terrestrial planets. Last but not least, images of protoplanetary disks with very high resolution (less than 10AU) are becoming possible with instruments such as ALMA and SPHERE. Our disk-planet models can be used to interpret these observations.
With MOJO, we have assembled a team with a range of expertise necessary to develop a model that explains both the giant planets of our Solar System and the diversity of planetary systems.
Monsieur Alessandro MORBIDELLI (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.
Lagrange (OCA/CNRS/UNS) Laboratoire J.L. Lagrange OCA/CNRS/UNS
LAB OASU CNRS_UB1 Laboratoire Astrophysique de Bordeaux
Help of the ANR 367,374 euros
Beginning and duration of the scientific project: September 2013 - 48 Months