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Grand discoveries shed light on the dynamics and evolution of multiple planetary systems

The path to the complete understanding of exoplanets is undoubtedly a long one. To help the scientific community solve remaining puzzles, an EU-funded project is studying the role of binary planetesimals in planet formation, exploring planetary satellites and investigating the underlying physical processes involved in the assemblage of exoplanet systems.

The GRAND (GRAvitational N-body Dynamics: Dynamics and evolution of multiple planetary systems) project has two core objectives: understanding the formation and evolution of planetary satellites, multiple moon systems and binary planetesimals and their role in planet formation; and characterising the properties and evolution of multiple-planet systems. Although it runs until February 2017, the project has already come a long way. Some of its most interesting results so far include the finding that impactors on Earth-like planets could have a very similar composition to the planets they impact, a detailed analysis of the order of the planets in multiple planetary systems, and stability criteria for moon survival in multi-planet systems. Prof. Hagai Perets, coordinator of GRAND and Assistant Professor at the Physics Department of Technion in Israel, outlines the project results so far. Why did you decide to focus your research on multiple planetary systems? Among the thousands of new exoplanets discovered over the past few years, a large number are multiple-planet systems, and it’s very likely that many of the ‘single’ planet systems have additional, but as yet undetected companions. Essentially, any attempt to understand planet formation and the structure of exoplanet systems has to deal with multiple-planet systems. In light of my expertise in stellar and planetary dynamics, this field is a natural choice given its important and rich physics. What is the specific benefit of studying the evolution of such systems? The interaction between planets and moons in planetary systems plays several important roles in the formation and their respective growth through collisions, as well as in shaping their structure. These interactions occur over both the short and very long term and change the orbits of the planetary objects. Understanding these interactions is therefore crucial in explaining the origins and characteristics of exoplanets and of our own Solar system. What kind of data did you use for your research? My research is mostly theoretical and I use both analytic and simulation tools to model the evolution of planetary systems. In terms of data, I use both data from simulations as well as data from observations, most notably data obtained from the Kepler mission for transit detection of planets, missions exploring the Solar system and its moons, as well as data from ground telescopes. You notably solved an old problem regarding the Earth-Moon composition similarity. Can you elaborate on that? The origin of moons had been debated over the previous century. The main paradigm over the past 40 years has set out that the Moon was formed following a giant impact of a Mars-like object with the proto-Earth. These models had proved successful in explaining most of the properties of the Moon and the Earth-Moon system. However one main challenge could not be overcome — the composition problem. It was found that the isotopic composition of the Earth and the Moon are very similar. However, simulations of giant impacts showed that most of the material eventually forming the Moon had to come from the impactor and not the Earth itself, as this is the case for other planetary objects in the Solar system such as Mars and the asteroid Vesta. This problem had become even more acute as improved composition measurements showed how similar the Earth and the Moon are. In my research I have reconsidered some of the basic assumptions in this logic, and in particular I asked the question of whether the composition of impactors is as different as that of other non-impacting planets in a Solar system. We used data from dozens of extensive simulations of Solar system-like formations and studied the compositions of the planets and those of Earth-like impactors. We found that although different planets had different compositions, the composition of impactors (just before the impact) was much more similar to the planet they hit. Moreover, in a non-negligible fraction of the cases, we found that there were as many composition similarities between the Moon-like object expected to form and the planet they impacted as between the Earth and the Moon. In other words, we showed that the 40-year old composition problem might not be a problem at all, and that the giant-impact hypothesis can overcome it. How do you hope your study of the order of the planets in exoplanet systems will contribute to future theoretical predictions? One of the projects I'm working on is exploring the order of planets in exoplanet systems — if we have three planets of different sizes we could have six permutations of how to order them, for example. The order of the planets is a result of a complicated evolution, much like other properties of the planets like the ellipticity (eccentricity) of their orbits or the distribution of their size. The latter two serve as important properties where distribution can be used to constrain the formation processes of the planets. As such they are studied extensively. In my research I try to advance the notion that the order of the planets is another property which has hitherto been mostly ignored, but can convey as much if not more information than the other, heavily studied, properties. Our preliminary results show that the properties of the order of the planets are not trivial and are not consistent with many of the current predictions — hence they provide new observational constraints on theories for the formation of exoplanets. Other than that, what would you say are the most important things you have learned from the research so far? Among other things I devised a completely novel method for analysing data from the Kepler mission so as to determine the distribution of the inclination between exoplanets’ orbits and their host star — an important property for understanding their evolution. This new method provided for the first time large-scale statistical properties of the inclination distribution and its relation to planets’ sizes, distances and multiplicity, something that could not be obtained with any other existing method. The project ends in February 2017. What do you hope will be its overall impact when you've achieved all of your objectives? I hope the project will shed new light and open up the study on the order of multi-planet systems, and hence relate and correlate the properties of different planets in the same system — an issue which has rarely been explored so far. I also hope to change our views of satellite systems and their formation and dynamics, with respect to both the Earth-Moon system (which I further explore) and other Solar system moons. GRAND Funded under FP7-PEOPLE CORDIS project page