Effects of capturing a wide-orbit planet on planetary systems: system stability and habitable zone bombardment rates

A large fraction of stars are formed in dense clusters. In the cluster, close encounters between stars at distances less than 100 au are common. It has been shown that during close encounters planets can transfer between stars. Such captured planets will be on different orbits compared to planets formed in the system, often on very wide, eccentric, and inclined orbits. We examine how these captured planets affect Kuiper belt-like planetesimal belts in their new systems by examining the effects on habitable planets in systems containing an outer gas giant. We show that these captured planets can destabilize the belt, and we show that the fraction of the planetesimals that make it past the giant planets into the system to impact the habitable planet is independent of the captured planet's orbital plane, whereas the fraction of the planetesimals that are removed and the rate at which they are removed depend strongly on the captured planet's pericentre and inclination. We then examine a wide range of outcomes of planet capture and find that when a Jupiter-mass planet is captured it will in 40 per cent of cases destabilize the giant planets in the system and in 40 per cent of cases deplete the belt in a few Myr, i.e. not posing much risk to life on terrestrial planets that would be expected to develop later. In the final 20 per cent of cases, the result will be a flux of impactors 10-20 times greater than that on Earth that can persist for several Gyr, detrimental to the development of life on the planet.