Planets in Double Stars

Stability studies of planetary motion in binary systems are very important, since we expect that more and more planets in such stellar systems will be discovered in the future - due to the fact that most of the stars in the solar neighborhood form double or multiple star systems. Currently (January 2007) we know only few planetary systems, which have a "close" stellar companion, out of a - meanwhile long - list of extrasolar planetary systems. The total number of binaries with planets is about 30. In most of these double-star systems the distance of the two stellar components is between 100 and more than 6000 AU. Therefore, it is obvious that the detected planets were found to move around one stellar component. This chapter gives an overview about planetary orbits in binary systems, where the different types of motion are discussed. First, we present some general stability studies that define the stable region for planetary motion in such systems, where different mass ratios of the stellar components were considered. In this context we discuss the influence of both eccentricities - that of the binary and that of the planet. We show that circular planetary motion cannot always give the necessary information, whether a planet is in a stable zone or not, especially when high eccentric planetary orbits are close to the border of stable motion. Some applications to real binary systems will underline the importance of such general investigations. Moreover, we show some results about the influence of the secondary companion on the region between the host star and the giant planet.