The effect of dynamical interactions in stellar birth environments on the orbits of young close-in planetary systems

TL;DR

This study uses N-body simulations to demonstrate how stellar fly-bys in clustered birth environments can significantly alter the architecture of young close-in planetary systems, affecting their long-term stability and transit observability.

Contribution

It provides the first detailed analysis of how single stellar encounters can modify the architecture of close-in exoplanetary systems over hundreds of millions of years.

Findings

Stellar fly-bys can reduce the number of observable transiting planets.

Fly-bys can induce high mutual inclinations, affecting transit detection.

The results may explain the excess of single-transit systems observed.

Abstract

Stars do not form in isolation but together with other stars, and often in a clustered environment. Depending on the initial conditions in these environments, such as initial density and substructure, the distances of encounters between stars will differ. These encounters can also affect just-formed exoplanetary systems. Using N-body simulations, we show the effect of a single fly-by on a common type of exoplanetary system: close-in Super-Earths/sub-Neptunes with or without a distant Giant planet. Even a single encounter can significantly modify the architecture of these exoplanetary systems over their long lifetimes. We test fly-bys with different characteristics, such as distance and mass, and show how they perturb the inner planets long after the encounter, leading to collisions and mutual inclination excitation, which can significantly modify the observed architecture of these systems in transit. We find that our initially four-planet inner systems reduce to three or two inner planets depending on their initial separation and that the mutual inclinations of these remaining planets can be high enough to reduce the number of observable, transiting planets. In our 500 Myr simulations, we show that this reduction in the number of transiting planets due to stellar fly-bys can contribute to the observed excess of single-transit systems.