Capture of satellites during planetary encounters A case study of the Neptunian moons Triton and Nereid

TL;DR

This study uses N-body simulations to explore how planetary encounters can lead to satellite capture, explaining the origins of Triton and Nereid and predicting exomoon transfer between planets.

Contribution

It demonstrates that satellite capture during planetary encounters can produce diverse moon orbits, including Triton-like and Nereid-like objects, with implications for exoplanetary systems.

Findings

Capture probability reaches ~10% during close encounters.

Captured moons can be circularized or remain eccentric, resembling Triton and Nereid.

Exomoons can transfer between planets during planetary scattering.

Abstract

Single-binary scattering may lead to an exchange where the single object captures a component of the binary, forming a new binary. This has been well studied in encounters between a star--planet pair and a single star. Here we explore the application of the exchange mechanism to a planet--satellite pair and another planet in the gravitational potential of a central star. As a case study, we focus on encounters between a satellite-bearing object and Neptune. We investigate whether Neptune can capture satellites from that object and if the captured satellites have orbits analogous to the Neptunian moons Triton and Nereid. Using $N$-body simulations, we study the capture probability at different encounter distances. Post-capture, we use a simple analytical argument to estimate how the captured orbits evolve under collisional and tidal effects. We find that the average capture probability reaches $\sim$$10\%$ if Neptune penetrates the donor planet's satellite system. Most moons grabbed by Neptune acquire highly eccentric orbits. Post-capture, around half of those captured, especially those on tight orbits, can be circularised, either by tides only or by collisions+tides, turning into Triton-like objects. Captures further out, on the other hand, stay on wide and eccentric orbits like that of Nereid. Both moon types can be captured in the same encounter and they have wide distributions in orbital inclination. Therefore, Triton naturally has a $\sim$50\% chance of being retrograde. A similar process potentially applies to an exoplanetary system, and our model predicts that exomoons can jump from one planet to another during planetary scattering. Specifically, there should be two distinct populations of captured moons: one on close-in circular orbits and the other on far-out eccentric orbits. The two populations may have highly inclined prograde or retrograde orbits.