Eccentricity growth and orbit flip in coplanar hierarchical three body systems

The dynamical evolution of a hierarchical three body system is well characterized by the eccentric Kozai-Lidov mechanism, where the inner orbit can undergo large eccentricity and inclination oscillations. It was shown before that starting with a circular inner orbit, large mutual inclination (40-140 degree) can produce long timescale modulations that drives the eccentricity to extremely large value and can flip the orbit. Here, we demonstrate that starting with an almost coplanar configuration, for eccentric inner and outer orbits, the eccentricity of the inner orbit can still be excited to high values, and the orbit can flip by ~180 degree, rolling over its major axis. The ~180 degree flip criterion and the flip timescale are described by simple analytic expressions that depend on the initial orbital parameters. With tidal dissipation, this mechanism can produce counter-orbiting exo-planetary systems. In addition, we also show that this mechanism has the potential to enhance the tidal disruption or collision rates for different systems.