# Creating retrogradely orbiting planets by prograde stellar fly-bys

## TL;DR

This paper demonstrates that stellar fly-bys, including prograde ones, can induce retrograde orbits in planets, providing a new mechanism for the formation of misaligned and retrograde exoplanets.

## Contribution

The study reveals that even prograde stellar fly-bys can produce retrograde planetary orbits, expanding the understanding of planet-star orbital misalignments.

## Key findings

- Prograde fly-bys can generate retrograde planets with about 20% probability.
- Close, coplanar, equal-mass fly-bys are most effective in producing retrograde orbits.
- Inclined fly-bys are more likely to result in retrograde planets than less inclined ones.

## Abstract

Several planets have been found that orbit their host star on retrograde orbits (spin-orbit angle {\phi} > 90{\deg}). Currently, the largest measured projected angle between the orbital angular momentum axis of a planet and the rotation axis of its host star has been found for HAT-P-14b to be $\approx$ 171{\deg}. One possible mechanism for the formation of such misalignments is through long-term interactions between the planet and other planetary or stellar companions. However, with this process, it has been found to be difficult to achieve retrogradely orbiting planets, especially planets that almost exactly counter-orbit their host star ({\phi} $\approx$ 180{\deg}) such as HAT-P-14b. By contrast, orbital misalignment can be produced efficiently by perturbations of planetary systems that are passed by stars. Here we demonstrate that not only retrograde fly-bys, but surprisingly, even prograde fly-bys can induce retrograde orbits. Our simulations show that depending on the mass ratio of the involved stars, there are significant ranges of planetary pre-encounter parameters for which counter-orbiting planets are the natural consequence. We find that the highest probability to produce counter-orbiting planets ($\approx$ 20%) is achieved with close prograde, coplanar fly-bys of an equal-mass perturber with a pericentre distance of one-third of the initial orbital radius of the planet. For fly-bys where the pericentre distance equals the initial orbital radius of the planet, we still find a probability to produce retrograde planets of $\approx$ 10% for high-mass perturbers on inclined (60{\deg} < i < 120{\deg}) orbits. As usually more distant fly-bys are more common in star clusters, this means that inclined fly-bys probably lead to more retrograde planets than those with inclinations < 60{\deg}. (...)