Capture of satellites during planetary encounters

Abstract

Context. 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. Aims. 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. Methods. 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. Results. We find that the average capture probability reaches ~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 ~50% chance of being retrograde. Conclusions. 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.

The spectroastrometric detectability of nearby Solar System-like exomoons

“…The detection and characterisation of such satellites accompanying extrasolar planets holds the potential to further our understanding of planet formation, evolution, and habitability. Different moon and planet formation scenarios and their outcomes have been linked to predictions of or requirements on, for example, the time of formation of the satellites (Cilibrasi et al 2018), their size and mass relative to their host (Nakajima et al 2022;Canup & Ward 2006;Hansen 2019), circumplanetary disc (CPD) composition (Batygin & Morbidelli 2020;Oberg et al 2023), host magnetosphere (Canup & Ward 2006), host mass (Oberg et al 2023), instellation or host migration history (Heller & Pudritz 2015a,b), and orbital properties (Li et al 2020). While the Solar System is host to a large number of moons, the limited scenario we are presented with cannot conclusively bear witness to any of these analyses: validation of these formation studies would require the detection and characterisation of satellites of planets outside the Solar System.…”