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Building planetary systems: linking architectures with formation

Periodic Reporting for period 3 - BuildingPlanS (Building planetary systems: linking architectures with formation)

Reporting period: 2019-06-01 to 2020-11-30

For hundreds of years our understanding of how planets form was limited to only the Solar System, but since the discovery of the first extra-solar planets (in the 1990s) our knowledge of planetary systems has increased at a startling rate; we now know of thousands of exoplanet systems. There have been many unexpected discoveries, but the biggest surprises was that most of the systems we observe look nothing like our own: we see “hot Jupiters” which orbit their stars in just a few days, planets which meander across entire solar systems on highly eccentric orbits, compact systems with five or six giant planets in tightly-packed short-period orbits, and even planets orbiting twin, binary suns. However, this huge advance in our knowledge has not yet led to a corresponding increase in our understanding, and many aspects of the planet formation process remain a mystery. The BuildingPlanS project aims to understand the origins of the enormous diversity we see in exoplanet architectures, by relating the properties of exoplanet systems to their formation in cold discs of dust and gas orbiting around young, newly-formed stars.
"Our team has made several advances in our understanding of how young planets interact with their parent protoplanetary discs. We have developed new computer models of systems containing giant planets, which have sufficient mass to open gaps in their parent discs. Although these young planets are mostly invisible to our telescopes their effect on the discs can be observed, and we have shown how new data can be used to understand how young giant planetary systems form and evolve. We have also built sophisticated computer simulations of multi-planet systems, in order to understand how the gravitational interaction between planets can shape the formation and evolution of these systems. In theoretical models pairs of planets often become ""trapped"" in special locations, known as resonances, where the planets' mutual gravity has a strong effect, but very few of these resonances are seen in real systems. Our calculations have shown how the protoplanetary disc can push planets out of resonance, and these calculations will have important implications for our understanding of how observed ""super-Earths"" and Neptune-size planets were formed. We have also studied of planets on inclined discs, and planets and discs in binary systems, using large-scale computer simulations to model the dynamics, evolution, and observational appearance of these systems."
Planet formation spans a vast range in scales, but the intermediate stages of the process - everything from metre-size rocks up to Earth-size planets - are essentially invisible to our telescopes. As a result, theoretical models and computer simulations are the only way we can link the early stages of planet formation (in protoplanetary discs) with mature exoplanet systems. We have already made important steps forward in understanding these early phases; the next stage of the project is to extend our work to consider the evolution of large numbers of planetary systems. This will allow us to look at the properties of forming planetary systems statistically, in order to build up a comprehensive picture of the processes that shape planetary systems.
Numerical model and ALMA observation of the young star Elias 24, which hosts a forming planet.