Institute for Advanced Study Informal Astrophysics Seminar

Though ~400 planets are currently known, discoveries at the observational frontiers continue to yield new classes of planets. The three gas giants directly-imaged orbiting HR 8799 comprise the first multi-planet system detected at wide separations around a main sequence star. Core accretion scenarios, already strained at the outer limits of our solar system, have difficulty explaining these objects. Though most plausible for massive planets at large separations, formation by gravitational instability requires that the system's protoplanetary disk passed through a fine-tuned region of parameter space. Orbital stability requirements imply that the HR 8799 planets occupy at least one and possibly two mean motion resonances, suggesting that they migrated toward one another and may have migrated substantially from their formation locations. I will describe how wide separation giants inform our theoretical understanding of the formation and evolution of planetary systems.