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GRAvitational N-body Dynamics:
Dynamics and evolution of multiple planetary systems

Final Report Summary - GRAND (GRAvitational N-body Dynamics:Dynamics and evolution of multiple planetary systems)

The main focus of the proposed research is understanding the formation and evolution of few and many body planetary gravitating systems. These include a wide range of scales, from planet formation and the evolution of planetesimals and binary planetesimals (BPs; including the dynamical evolution of asteroids, Kuiper belt objects) to multiple moon systems in the Solar system to the evolution of multiple-exoplanet systems (MPSs). Though different in scales, there are many parallels between the evolution and dynamics of these various type of planetary
systems, and some of the physical processes governing their behaviour can be very similar. Much of the underlying physics is therefore common, and similar tools can be used to explore them. I am trying to understand both the basic processes and the microphysics underlying the behaviour of these systems (e.g. close encounters, secular evolution, resonances, gas-particle interaction etc.), and their overall structure and evolution (e.g. statistical properties; velocity and radial distribution, collective processes). In this project explore novel directions in the study of realistic high multiplicity gravitating planetary systems, focusing both on purely gravitating systems as well as coupling of dynamical evolution with dissipative interactions between the gravitating bodies and their environments.
The two main objectives of the research are as follows:
1. Understanding the formation and evolution of planetary satellites, multiple moon systems and binary planetesimals and their role in planet formation.
2: Characterizing the properties and evolution of multiple-planet systems, and using them to probe planet formation and planetary dynamics processes

Since the beginning of this project we have worked in parallel on several directions, both on the theoretical side, as well as on the analysis of observational data.
In order to understand the evolution of binary planetesimals we explored both their evolution through scattering and collision with other planetesimals in a protoplanetary disk (using N-body simulations), as well as the dissipative effects of gas-drag on the evolution of single and binary planetesimals. Our scattering N-body experiments are mostly complete, and their will likely be finalized by the end of the year. Our studies of the effects of gas-planetesimals interactions revealed the importance of gas-dynamical friction in significantly affecting the orbits and evolution of single and binary planetesimals, leading to orbit circularization, rapid migration and binary-planetesimals mergers. These results are summarized in two papers, both published (Grishin & Perets, 2015, 2016).

We have also studied the evolution of multiple-moon systems around the gas giants, as well as in Pluto-Charo system, as well as the stability of moons in multi-planet systems. We derived stability criteria for moons survival in multi-planet systems (published;Payne et al. 2014) and showed that mutual scattering could lead to excitation of the moon into inclined and eccentric orbits, thereby producing irregular moons. We analyzed the dynamics and stability of moons in the Pluto-Charon system (published; Michaely, Perets & Grishin 2017), and derived the first analytical general stability criteria for binary planetesimals and satellites at arbitrary inclinations (published; Grishin, Perets, Zenati & Michaely 2017) and we are also exploring the evolution of gas-giant moon system when migration and accretion are included, as well as the effect of planetary oblateness; these studies are still in progress.

We have explored the evolution of a planetesimal disk with many planetary embryos that produce Solar-system-like planetary systems. We analyzed the data and made use of the data from tens of simulations to solve a decades old problem regarding the Earth-Moon composition similarity. We found that impactors on Earth-like planets could have very similar composition to the planets they impact, contrary to previous belief, thereby alleviating one of the main challenges to the giant-impact model for the Moon formation. These were published in Nature journal (Mastrobuono-Battisti, Perets & Raymond 2015), and in MNRAS (Mastrobuono-Battisti & Perets 2017). In addition I proposed a novel multiple-impact origin for Earth’ moon, and showed its applicability in a paper published in Nature Geoscience with my collaborators (Rufu, Aharonson & Perets 2017)

We have run a multitude of multiple-planet N-body simulations as well as secular evolution studies and gathered significant statistics, which we currently analyze, and which will allow us to provide robust understanding of the dynamics of such systems and their outcomes. We already have several novel conclusions. In particular we show that multiple-planet systems in binary-star systems realign themselves to the binary orbit, irrespective of the initial inclination. We also find evidence for novel eccentricity-inclination ad eccentricity-seminar major axis correlations in such systems not reported before elsewhere. Unfortunately, the student working on these issue left science after his Msc, and we therefore postpone further analysis and publication of these data to the next year, after recruiting another student to work on this project.
The secular dynamic studies have been done by another student of mine (Adrian Hamers, co-supervised with Simon Portegies-Zwart from Leiden) leading to three relevant publications, explaining the lack of short-period circumbinary planets, as well as exploring the secular chaos origin of hot Jupiter exoplanets (Hamers et al. 2015, 2016, 2017).

On the observational side we have analyzed data of the order of the planets in multiple-planet systems both found through radial velocity measurements as well as through transit data (Kepler mission). We have for the first time a detailed analysis of the order of the planets in >2 planetary systems, which can be directly compared to the data we have collected from simulations. Both the the analysis of the observational data as well as their comparison with theory will compiled and submitted for publication in the coming year. We have also used analysis of photometric data and proposed a novel photometric-only method to study the spin-orbit properties of planetary systems (published; Mazeh, Perets, Mcquillan & Goldstein 2015). Follow-up paper is now under preparation and will be submitted in the coming few months.

The PI website can be found in