Statistical properties of exoplanets - III. Planet properties and stellar multiplicity

Among the hundred or so extrasolar planets discovered to date, 19 are orbiting a component of a double or multiple star system. In this paper, we discuss the properties of these planets and compare them to the characteristics of planets orbiting isolated stars. Although the sample of planets found in multiple star systems is not large, some differences between the orbital parameters and the masses of these planets and the ones of planets orbiting single stars are emerging in the mass-period and in the eccentricity-period diagrams. As pointed out by Zucker & Mazeh (2002), the most massive short-period planets are all found in multiple star systems. We show here that the planets orbiting in multiple star systems also tend to have a very low eccentricity when their period is shorter than about 40 days. These observations seem to indicate that some kind of migration has been at work in the history of these systems. The properties of the five short-period planets orbiting in multiple star systems seem, however, difficult to explain with the current models of planet formation and evolution, at least if we want to invoke a single mechanism to account for all the characteristics of these planets.