Dynamics and planet formation at wide separations: HR8799
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 planet formation at the larger distances of the HR 8799 planets, even around a central A star. 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 at its transition from the Class I to the Class II phase. 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 discuss how the HR 8799 system constrains the formation and migration of distant planets.