The dynamical evolution of multiplanet systems in open clusters

The majority of stars form in star clusters and many are thought to have planetary companions. We demonstrate that multiplanet systems are prone to instabilities as a result of frequent stellar encounters in these star clusters much more than single-planet systems. The cumulative effect of close and distant encounters on these planetary systems are investigated using Monte Carlo scattering experiments. We consider two types of planetary configurations orbiting Sun-like stars: (i) five Jupiter-mass planets in the semimajor axis range 1-42 au orbiting a Solar mass star, with orbits that are initially coplanar, circular and separated by 10 mutual Hill radii and (ii) the four gas giants of our Solar system. We find that in the equal-mass planet model, 70 per cent of the planets with initial semimajor axes a > 40 au are either ejected or have collided with the central star or another planet within the lifetime of a typical cluster, and that more than 50 per cent of all planets with a < 10 au remain bound to the system. Planets with short orbital periods are not directly affected by encountering stars. However, secular evolution of perturbed systems may result in the ejection of the innermost planets or in physical collisions of the innermost planets with the host star, up to many thousands of years after a stellar encounter. The simulations of the Solar system-like systems indicate that Saturn, Uranus and Neptune are affected by both direct interactions with encountering stars, as well as planet-planet scattering. Jupiter, on the other hand, is almost only affected by direct encounters with neighbouring stars, as its mass is too large to be substantially perturbed by the other three planets. Our results indicate that stellar encounters can account for the apparent scarcity of exoplanets in star clusters, not only for those on wide-orbit that are directly affected by stellar encounters, but also planets close to the star which can disappear long after a stellar encounter has perturbed the planetary system.