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New Frontiers in Modeling Planet-Disk Interactions: from Disk Thermodynamics to Multi-Planet Systems

Periodic Reporting for period 1 - DiskTorqueOnPlanets (New Frontiers in Modeling Planet-Disk Interactions: from Disk Thermodynamics to Multi-Planet Systems)

Reporting period: 2017-09-01 to 2019-08-31

For thousands of years, humankind had only been aware of the handful of planets composing our Solar System. This changed dramatically over the last two decades with the discovery of several thousand planets around nearby stars. Most of these, so-called exoplanets, show remarkable differences when compared to the Solar System. We believe that processes leading to the formation of planets, combined with environmental effects associated with the protoplanetary disk in which they form, play a fundamental role in sculpting planets and planetary systems. Thus, in order to understand the wide diversity of planets we observe, and contextualize our Solar System, it is critical to study and characterize such mechanisms self-consistently together with the dynamical evolution of the protoplanetary disk. One of the processes that may play an important role is the so-called planetary migration due to the mutual gravitational interaction between the planet and the disk. As a planetary embryo grows accreting material from the protoplanetary disk, it exerts a gravitational force onto the disk which, by the law of action-reaction, exerts an opposite force onto the planet. This force accelerates the planet and makes it move. So far, it is not clear observationally, whether migration due to planet-disk interaction occurs over large spatial scales during the formation of planetary systems. Identifying and characterizing the various mechanisms that set the speed and direction of planet migration demands a thorough understanding of the physics and dynamics governing protoplanetary disks and planets. In this context, the core of the project was to study the contribution of physical mechanisms that have been overlooked until now and could play a significant role in this story. One of the highlights of the project was the realization that dust (known to constitute only a small fraction of the protoplanetary disks mass) may play an important role in the early dynamical history of planetary embryos.
To study this problem in a variety of physical conditions, we developed numerical methods and implemented them in high-performance computer codes to run state-of-the-art numerical experiments. The main tool developed and employed in this project is the publicly available code FARGO3D. During the project, we extended this code to include more physics in order to address the problem of planet-disk interaction in more realistic settings. These tools have enabled us not only to discover and systematically study gravitational forces due to the dust component in protoplanetary disks but also to expand our research front to address fundamental questions related to dust-growth and dust-dynamics in protoplanetary disks. One of the key outcomes of our research is the development and implementation of a robust numerical algorithm to account for the drag force produced between the gas and dust particles present in a protoplanetary disk. This tool allowed us to find a number of interesting effects arising from the dust component in protoplanetary disks. For example, we found that planet migration of low-mass planets can be severely modified by considering the dynamics of solids in protoplanetary disks. Our tools have also been useful to characterize further the so-called Streaming Instability, an instability proved to develop in the disk because of the interaction between the dust and gas components. Also, the tools we developed have been exploited to characterize the transport of dust in protoplanetary disks in different situations, for example when a giant planet acts as a barrier (like Jupiter in the early Solar System) or under the presence of magnetic fields in the protoplanetary disk. All of the results we obtained during this project have been published in peer-reviewed journals and are available in the public repository arxiv.org https://arxiv.org. The modules developed are also publicly available (http://fargo.in2p3.fr/) to the benefit of the astronomical community worldwide.
When we started the project, it was hard to think that dust could have such an impact on planet migration. After realizing that this could indeed be the case, we had the urge to investigate several aspects of dust-dynamics in protoplanetary disks in ways that have never been attempted before. This motivated us to build-up a robust framework to study multispecies dust dynamics in protoplanetary disks. This tool constitutes now a solid foundation to build sophisticated self-consistent thermodynamic models of protoplanetary disks well beyond the state-of-the-art as defined at the moment this project started. The tools and ideas we have developed will have an impact well beyond the scope of our original project. Having developed algorithms and implemented new physics in the numerical code enables us not only to simulate and study how planetary systems evolve dynamically but also to explore more fundamental questions concerning the early stages of planet formation. For example, we found that the tools we have developed are very useful for studying processes that are believed to participate during the formation of planetesimals, like the so-called streaming instability. The key result is that a self-consistent treatment of dust-dynamics including multiple particle sizes matters not only for planet migration processes but for planet formation more generally. Taken together these results highlight the crucial need to understand the role of dust dynamics in the processes that shape the overall physical properties of the diverse new worlds we will continue to discover in the coming decades.
The most immediate beneficiary of the tools developed during the project is undoubtedly the large community of astrophysicists working intensively to understand how the revolutionary data being gathered by the Atacama Large Millimeter Array (ALMA) can help us unravel the processes that shape planetary systems. It is clear that in order to make the most of these observations it is necessary to develop and make available to the community, numerical codes to model the complex dynamical processes shaping protoplanetary disks as we observed them. In a broader context, the insights that this will enable have the potential to impact other communities including astrochemistry. On a broader scale, a significant improvement in our understanding and characterization of how dust moves and concentrates on protoplanetary disks has the potential to change the way we think planets form and evolve.
Protoplanetary disk with an embedded planet orbiting a central star