The Effects of Encounters on Planetary Systems in Stellar Clusters

Abstract: Extrasolar planets are found to be on orbits very different from those of the planets in the solar system. Gas-giant planets have been found on orbits tighter than that of the planet Mercury, often with very high eccentricities. These orbits are surprising; models of planet formation suggest that gas giants should be found on circular orbits with similar size to those of solar-system gas giants. In this thesis we describe how encounters between stars in young stellar clusters can increase the eccentricities of gas-giant planets on similar orbits to those of the gas giants in the solar system. We have measured how common encounters are, both so-called fly-bys and exchange encounters. In the latter, a single star is exchanged into a binary system, while in a fly-by two stars pass close to each other. If a single star is exchanged into a binary, the effect of the companion is to periodically increase the eccentricities and inclinations of the planets. This leads to the orbits of planets crossing, triggering planet-planet scattering. In a fly-by, the effect of the intruder star can be to immediately eject and/or capture one or more planets. More common, however, is that the fly-by only perturbs the orbits of the planets, which on time scales of a few to more than 100 million years triggers planet-planet scattering. The outcome of planet-planet scattering, whether triggered by the companion star in a binary or by the intruder star in a fly-by, is most often the ejection of one or more planets, leaving those remaining on more eccentric orbits. We find that encounters in stellar clusters will significantly alter the population of planetary systems orbiting stars in the cluster. As many stars form in some type of cluster or association, encounters between stars can thus play an important role in determining the properties of the observed extrasolar planet population.