Gravitational instability in binary protoplanetary disks

We review the models and results of simulations of self-gravitating, gaseous protoplanetary disks in binary star systems. These models have been calculated by three different groups with three different computational methods, two particle-based and one grid-based. We show that interactions with the companion star can affect the temperature distribution and structural evolution of disks, and discuss the implications for giant planet formation by gravitational instability as well as by core accretion. Complete consensus has not been reached yet on whether the formation of giant planets is promoted or suppressed by tidal interactions with a companion star. While systems with binary separations of order 100 AU or larger appear to behave more or less as in isolation, systems with smaller separations exhibit an increased or decreased susceptibility to fragmentation, depending on the details of thermodynamics, in particular on the inclusion or absence of artificial viscosity, and on the initial conditions. While code comparisons on identical problems need to be carried out (some of which are already in progress) to decide which computer models are more realistic, it is already clear that relatively close binary systems, with separations of order 60 AU or less, should provide strong constraints on how giant planets form in these systems.