The effect of close stellar flybys on the distant minor body population.

Introduction

GAIA mission astrometric data for stars in Solar System neighbourhood allow us to find a probable past and future stellar flybys at distance lower than 3000 au (for example HD 7977 flyby 2.5 million years ago). However the distance and geometry of such flybys have large relative uncertainties and it is possible that flybys at a distance of several hundreds of astronomical units have happened in recent past. We performed the numerical analysis of very close stellar flyby investigating different flyby geometries and distances. In the analysis we simulated the dynamics of planets and Kuiper Belt minor bodies during the passage of a Sun-like star using the Rebound n-body simulator software. Using the past flyby of HD 7977 star as a reference we assumed the star with mass close to the mass of the Sun and relative velocity of tens of km/s.

Before analysing the effect of star passage on minor bodies we decided to check the impact on the orbits of giant planets. We discovered that flybys at distance greater than 300 au change the eccentricity of giant planets by less than 0.01 and the semimajor axis by less than 0.1 au. This allows us to ensure that such close flyby can have small effect on planets and not disturb the stability of Solar System and therefore they could happened in the past. The probability of significant disturbance of planetary orbits grows when the minimal star-Sun distance is smaller, but even for extremely close distances (about 100 au) in most cases it is possible to find specific geometry of flyby where planetary orbits are not heavily changed.

Kuiper Belt and scattered disc simulation

In order to investigate the effect of stellar flyby on Kuiper Belt objects, we simulated the synthetic, randomly generated population of minor bodies with semimajor axis greater than 30 au. We checked the statistical properties properties of orbital elements of this population after the close flyby. The results shows that flyby at 3000 au will not create significant change in the orbital parameters of this objects. However the closer passages do affect the population and may create various statistical effects on the population.

Last years the analysis of the orbital parameter of most distant Solar System objects shows the significant perihelion direction clustering. This leads to the formulation of the Planet Nine hypothesis by Batygin et al. In this model, the orbital characteristic of distant objects is explained by the existence of an undiscovered massive planet that orbits the Sun at a distance of about 1000 au. Our simulations also included scattered disc objects including distant bodies with the semimajor axis equal to several hundreds of astronomical units. We want to show if the very close star passage can be used as an alternative explanation of the orbital properties of distant bodies. We also checked what would be the possible effect of a star flyby on hypothetical Planet’s Nine orbit.

Conclusion

Our simulation shows that there is possibility of a close stellar flyby which do not disturb the planetary system affecting its stability, but such flyby can affect the minor body population, especially the distant part of the Kuiper Belt. This possibility should be taken into account in modelling the outer Solar System minor body population.