Speaker: Sean Raymond (Bordeaux U.)
Title: 
Bombardments on distant worlds: an upper limit on late accretion and water delivery in the Trappist-1 system
Abstract:
The impact rates on Solar System planets and moons can be evaluated by counting the craters on planetary surfaces or by careful analysis of meteorites.  These methods are hopeless for exoplanets.  We demonstrate a novel but simple method to place upper limits on the bombardment experienced in "multi-resonant" systems, in which each pair of neighboring planets is in mean motion resonance. We focus on Trappist-1, a well-characterized seven-planet resonant chain in which all planets are close to Earth's size.  We use the system’s fragile orbital structure to place robust upper limits on the planets’ bombardment histories. We use N-body simulations to show how perturbations from additional objects can break the multi-resonant configuration by either triggering dynamical instability or simply removing the planets from resonance. The planets cannot have interacted with more than 5% of an Earth mass in planetesimals – or a single rogue planet more massive than Earth’s Moon – without disrupting their resonant orbital structure. This implies an upper limit of 10^-4 to 10^-2 Earth masses of late accretion on each planet since the dispersal of the system’s gaseous disk. This is comparable to or less than the late accretion on Earth after the Moon-forming impact, and demonstrates that the Trappist-1 planets’ growth was complete in just a few million years, roughly an order of magnitude faster than Earth’s. Our results imply that any large water reservoirs on the Trappist-1 planets must have been incorporated during their formation in the gaseous disk. This has important implications for the formation of the abundant population of `super-Earths' and 'sub-Neptunes'.