Fly-by encounters between two planetary systems I: Solar system analogues

Abstract

Stars formed in clusters can encounter other stars at close distances. In typical open clusters in the Solar neighbourhood containing hundreds or thousands of member stars, ten to twenty per cent of Solar-mass member stars are expected to encounter another star at distances closer than 100 au. These close encounters strongly perturb the planetary systems, directly causing ejection of planets or their capture by the intruding star, as well as exciting the orbits. Using extensive N-body simulations, we study such fly-by encounters between two Solar System analogues, each with four giant planets from Jupiter to Neptune. We quantify the rates of loss and capture immediately after the encounter, e.g., the Neptune analogue is lost in one in four encounters within 100 au, and captured by the flying-by star in one in twelve encounters. We then perform long-term (up to 1 Gyr) simulations investigating the ensuing post-encounter evolution. We show that large numbers of planets are removed from systems due to planet-planet interactions and that captured planets further enhance the system instability. While encounters can initially leave a planetary system containing more planets by inserting additional ones, the long-term instability causes a net reduction in planet number. A captured planet ends up on a retrograde orbit in half of the runs in which it survives for 1Gyr; also, a planet bound to its original host star but flipped during the encounter may survive. Thus, encounters between planetary systems are a channel to create counter-rotating planets, This would happen in around 1% of systems, and such planets are potentially detectable through astrometry or direct imaging.

Capture of satellites during planetary encounters

“…For example, it becomes 42% for q enc = 0.012 au and 93% at 0.024 au. This agrees with the case of planet capture during stellar encounters, where during a distant flyby, all planets lost (if any) from one star are captured by the other (Li et al 2019). Because the number of moons captured during encounters with q enc = 0.024 au is limited, we do not analyse them in detail.…”

Stellar clustering shapes the architecture of planetary systems

“…At higher densities, they often eject the planet altogether 114 . However, for initial semi-major axes of a p 1 au, stellar encounters can significantly alter exoplanet architectures even in clusters of moderate densities (∼ 100 stars pc −3 ) 66,115 . Recent simulations have tried to address this problem by taking a population synthesis approach 116 .…”

Fly-by encounters between two planetary systems II: Exploring the interactions of diverse planetary system architectures

“…The encounters between these planetary systems in a cluster can be studied either in a Monte Carlo approach in the sense that the encounters are directly generated according to the expected distribution (e.g., Laughlin & Adams 1998;Malmberg et al 2011;Hao et al 2013;Li et al 2019) or in a selfconsistent cluster simulation where thus the frequency and parameters of the flybys flybys are more accurately modelled (see Spurzem et al 2009;Cai et al 2017;van Elteren et al 2019;Fujii & Hori 2019, for instance). Here we follow Paper I and adopt the first method.…”

“…This probably results from the combination of the effect of the more closely-packed and more massive inner planets themselves and the contribution from the outer gas giant Kepler-48 e. That planet itself is more susceptible due to its larger stellar-centric distance. Just like Jupiter in the solar system (Hao et al 2013;Li et al 2019), it is indestructible in the post-encounter phase owing to its dominance in the systems's mass budget. The top panel of Figure 5 shows the stability of the system.…”