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More than half of stars reside in binary or multiple star systems and manyplanets have been found in binary systems. From theoretical point of view,however, whether or not the planetary formation proceeds in a binary system isa very complex problem, because secular perturbation from the companion starcan easily stir up the eccentricity of the planetesimals and causehigh-velocity, destructive collisions between planetesimals. Early stage ofplanetary formation process in binary systems has been studied by restrictedthree-body approach with gas drag and it is commonly accepted that accretion ofplanetesimals can proceed due to orbital phasing by gas drag. However, the gasdrag becomes less effective as the planetesimals become massive. Therefore itis still uncertain whether the collision velocity remains small and planetaryaccretion can proceed, once the planetesimals become massive. We performed {\itN}-body simulations of planetary formation in binary systems starting frommassive planetesimals whose size is about 100-500 km. We found that theeccentricity vectors of planetesimals quickly converge to the forcedeccentricity due to the coupling of the perturbation of the companion and themutual interaction of planetesimals if the initial disk model is sufficientlywide in radial distribution. This convergence decreases the collision velocityand as a result accretion can proceed much in the same way as in isolatedsystems. The basic processes of the planetary formation, such as runaway growthand oligarchic growth and final configuration of the protoplanets areessentially the same in binary systems and single star systems, at least in thelate stage where the effect of gas drag is small.

Formation of Protoplanets from Massive Planetesimals in Binary Systems