Heavy-Metal Jupiters by Major Mergers

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A team from the University of California-Berkeley seek to understand the origin of metal-rich Jupiters, asking specifically whether they can arise from collisions between smaller protoplanets. Some Jupiter-mass extrasolar planets contain as much as 100 Earth masses of 'metals', i.e. elements other than hydrogen and helium. Such 'heavy-metal' Jupiters are not expected from the conventional theory of gas giant formation, which predicts that a gas-dominated planet originates from hydrogen and helium accreting onto a solid core having only 10 Earth masses of rock and ice. This proposal asks whether metal-rich giants can arise from collisions between massive rocky cores. During the planet formation era, pairs of cores can successively collide and merge, with each merger doubling the metal content of the merger product. Such a 'giant impacts' phase of collisional growth has long been put forward to hold for rocky planets like the Earth; this project seeks to extend this model to gas giants like Jupiter. The project will simulate the merger history of forming planet cores as well as their growth through disk gas to reproduce the observed metal contents of gas giants. The work will provide context for how our Solar System’s Jupiter may have been shaped by collisions. The work will train a graduate student to join the scientific workforce, and train undergraduate research assistants with ambitions to teach the physical sciences. The investigators will partner with local K-12 programs and high school physics teachers to educate students and teachers in modeling and research. Concurrent mergers and gas accretion from the disk will be modeled with the open-source N-body code REBOUND, outfitted with routines for gas accretion and disk torques. A 'sub-grid' code that models protoplanet interiors will provide radii for use in REBOUND’s collision algorithm. Different prescriptions for planetary gas accretion, impact-generated atmospheric mass loss, and collisional mergers — perfect vs. hit-and-run, to be modeled with fast machine-learning algorithms — will be explored. The goal is to survey the range of possible outcomes in merger + gas accretion simulations, and to reproduce the mean trend and remarkably large scatter in the empirical mass-metallicity relation for giant planets. The work will also connect to varying extents with other exo-giant observables, including semi-major axes, eccentricities, multiplicities, luminosities, and spins.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.