Exocomets of β Pictoris

II. Two dynamical families of exocomets simulated with REBOUND

Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 June 2025
Accepted: 18 February 2026

Abstract

Aims. We investigate the dynamical evolution of particles in the β Pic system to determine likely formation pathways to the exocomet populations observed in the present day. We aim to relate these results to similar studies recently carried out since the discovery of the inner planet β Pic c.

Methods. We simulated the β Pic system using the non-symplectic adaptive N-body integrator IAS15 in REBOUND. We seeded the system with over 100 000 mass-less test particles that were evolved for 25 Myr. We adopted initial conditions and a particle distribution that closely match those used in similar simulations in recent works. Using IAS15, REBOUND resolves close encounters between test particles and the two gas giants in the system, which is crucial for understanding aspects of the dynamical evolution.

Results. Planet-disc interactions rapidly clear most of the system within 35 AU, apart from a region within the orbit of β Pic c and a region between 20 and 25 AU. After 10 Myr, exocomets can be sourced continuously from these regions, as well as from the inner edge of the region beyond ∼35 AU, where particles are stable on longer timescales. From the region interior to β Pic c, the exocomets are formed by excitation via mean-motion resonance with β Pic c, obtaining a narrow distribution of radial velocities, consistent with spectroscopic observations. Particles initialised in the outer system might enter onto star-grazing orbits, due to disruption by the two gas giants, causing a wider radial velocity distribution, and we propose that this population corresponds to a second dynamical family previously observed via spectroscopy. These particles typically undergo chaotic dynamical evolution for 10²−10³ years after passing the water sublimation limit at ∼8 AU, before reaching the sublimation distance of calcium near 0.4 AU, implying that the two families of exocomets could have different volatile contents. This dynamical simulation depends on the assumed initial conditions, including the system parameters and the morphology of the initial disc, precluding accurate predictions of the absolute rates at which particles are generated from within and from the exterior of the orbits of the two gas giants. However, we predict that it is a general property of the system that a fraction of particles that originates from outside of the orbit of β Pic b can be scattered onto star-grazing orbits.