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Anders Johansen. Profile picture.

Anders Johansen


Anders Johansen. Profile picture.

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


  • Daohai Li
  • Anders Johansen
  • Alexander J. Mustill
  • Melvyn B. Davies
  • Apostolos A. Christou

Summary, in English

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.


  • Lund Observatory
  • eSSENCE: The e-Science Collaboration

Publishing year





Astronomy and Astrophysics



Document type

Journal article


EDP Sciences


  • Astronomy, Astrophysics and Cosmology


  • Celestial mechanics
  • Planets and satellites: dynamical evolution and stability
  • Planets and satellites: formation
  • Planets and satellites: individual: Triton, Nereid




  • IMPACT: Comets, asteroids and the habitability of planets
  • Wallenberg Academy Fellow Project
  • PLANETESYS: The next-generation planet formation model


  • ISSN: 0004-6361