Close stellar encounters kicking planets out of habitable zone in
  various stellar environments
  Continuous habitability of a planet is a critical condition for advanced
forms of life to appear, but it can be endangered by astronomical events such
as stellar encounters. The purpose of this study is to analyze close stellar
encounters able to change planetary orbits initially in circumstellar habitable
zones and to investigate the expected encounter rates in a variety of stellar
environments. Using gravitational simulations for three-body systems, this
study analyzed the dependencies of encounter impact-parameters with kinematic,
geometric, and habitability parameters of the system. We also used kinematic
properties of various stellar regions and estimated encounter rates of the
events. The expected number of threatening stellar encounters in the Solar
neighborhood is $\approx3.9\times10^{-4}$ in 4 billion years, while for the
Galactic bulge environment, we expect approximately 5.5 times the value. The
encounter rates for other stellar environments are calculated and spheroidal
dwarf galaxies and globular clusters encounter rates are estimated. The results
show that in contrast with the solar neighborhood, close stellar encounters can
play a significant role in the expected number of planets with continuous
habitability in dense stellar environments. Another notable result shows that
threatening stellar encounter rate follows the number density of stars, and is
not strongly dependent of the region's velocity dispersion. Further
investigations are needed to study long-term multiple planetary systems and how
they can change the overall expected value of continuously habitable planets.
