Collaborative Proposal: Pebbles and Gas - The Supply Chain for Compact Planetary Systems

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The most common type of planet known in the universe occurs in what are known as Systems with Tightly-Packed Inner Planets. These planets have masses equal to or somewhat larger than the Earth's mass, and orbits around their stars that are well ordered. The planets likely formed in or near their present locations. The investigators have proposed a theory called Inside-Out Planet Formation that suggests that high pressure in the inner regions of debris disks forces pebbles, gas, and dust to group together in a ring. When these materials first form a planet, the planet sweeps out leftover pebbles to make an empty space in its orbit around its star. Then, a new ring of pebbles forms outside the planet. This process repeats, creating a system of planets around the parent star. The investigators will study three aspects of the supply of material that forms the planets under this theory: they will look for forces that would interrupt the flow of pebbles and dust in the disk; they will study hydrogen and helium gas addition to these early planets; and finally they will consider magnetic effects on this process. This project serves the national interest by improving our understanding of how planetary systems like our Solar System formed and evolved. The investigators will include this research topic in their teaching activities, and train a graduate student and post-doctoral researcher.Systems with Tightly-packed Inner Planets (STIPs) could host the most common type of planets in the universe. These planets have masses ranging from Earth to super-Earth, and well-aligned orbits. These properties argue for formation near their present locations. The "Inside-Out Planet Formation" theory addresses the properties of STIPs. It involves the sequential formation of planets from "pebble"-rich rings in inner regions of protoplanetary disks. Pebbles, formed from dust coagulation, migrate to the inner disk due to gas drag and collect at a local pressure maximum. Once the first planet forms from the pebble ring, it grows to open a gap. A new pebble ring starts to form exterior to the planet, and the process repeats. The STIP planets have a wide range of densities, implying some are surrounded by H/He atmospheres with up to ~10% of the planet's mass. However, they did not suffer runaway accretion of H/He to become gas giants. This research will investigate three aspects of the supply chain of material to and from the STIP's planets during both their formation and subsequent evolution. First, the investigators will study the radial inward drift of pebbles through the disk plane, focusing on streaming instabilities that could disrupt pebble drift by forming massive planetesimals. Second, they will study H/He gas accretion to terrestrial planets of sub-Earth to super-Earth masses. Third, they will calculate magnetically-regulated atmospheric evaporation rates of STIPs. The investigators will include this research topic in their teaching activities, and train a graduate student and post-doctoral researcher.