  Most of the observed extrasolar planets are found on tight and often
eccentric orbits. The high eccentricities are not easily explained by
planet-formation models, which predict that planets should be on rather
circular orbits. Here we explore whether fly-bys involving planetary systems
with properties similar to those of the gas giants in the solar system, can
produce planets with properties similar to the observed planets. Using
numerical simulations, we show that fly-bys can cause the immediate ejection of
planets, and sometimes also lead to the capture of one or more planets by the
intruder. More common, however, is that fly-bys only perturb the orbits of
planets, sometimes leaving the system in an unstable state. Over time-scales of
a few million to several hundred million years after the fly-by, this
perturbation can trigger planet-planet scatterings, leading to the ejection of
one or more planets. For example, in the case of the four gas giants of the
solar system, the fraction of systems from which at least one planet is ejected
more than doubles in 10^8 years after the fly-by. The remaining planets are
often left on more eccentric orbits, similar to the eccentricities of the
observed extrasolar planets. We combine our results of how fly-bys effect
solar-system-like planetary systems, with the rate at which encounters in young
stellar clusters occur. For example, we measure the effects of fly-bys on the
four gas giants in the solar system. We find, that for such systems, between 5
and 15 per cent suffer ejections of planets in 10^8 years after fly-bys in
typical open clusters. Thus, encounters in young stellar clusters can
significantly alter the properties of any planets orbiting stars in clusters.
As a large fraction of stars which populate the solar neighbourhood form in
stellar clusters, encounters can significantly affect the properties of the
observed extrasolar planets.
