DIPSY: A new Disc Instability Population SYnthesis

II. The Populations of Companions Formed Through Disc Instability

¹ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
² Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³ Department of Astrophysics, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

^★ Corresponding author: oliver.schib@unibe.ch

Received: 4 July 2025
Accepted: 1 September 2025

Abstract

Context. Disc instability (DI) might provide an explanation for the formation of some observed exoplanets. At the same time, our understanding of this top-down formation mechanism remains limited. Existing studies have made strong simplifications, and the predicted population is poorly known.

Aims. We aim at overcoming several limitations and produce a more advanced synthetic population of companions formed via DI that can be used for quantitative statistical comparisons with observations, and to make predictions for future surveys.

Methods. We applied the global end-to-end model described in Paper I of this series to perform a population synthesis of companions formed via DI. By using initial conditions compatible with both observations and hydrodynamical simulations, and by studying a large range of primary masses (0.05 M_⊙ to 5 M_⊙), we can provide quantitative predictions of the outcome of DI.

Results. In the baseline population, we find that ~10% of the discs fragment, and about half of these end up with a surviving companion after 100 Myr. Based on their mass, 75% of the companions are in the brown dwarf regime, 15% are low-mass stars, and 10% planets. At distances larger than ~100 AU, DI produces planetary-mass companions on a low percent level. Inside of 100 AU, however, planetary-mass companions are very rare (low per mill level). The average companion mass is ~30 M_♃ scaling weakly with stellar mass. Very few companions of all masses reside inside of 10 AU; outside this distance, the distribution is approximately flat in log-space. Eccentricities and inclinations are significant, with averages of 0.4 and 40^°. In systems with surviving companions, there is either one (80%) or two (20%) companions. The fraction of surviving synthetic brown dwarfs is consistent with observations, while that of planets is lower than observed. Most of the initial fragments do not survive on a Myr timescale; they either collide with other fragments or are ejected, resulting in a population of free-floating objects (about 1–2 per star). We also quantify several variant populations to critically assess some of our assumptions used in the baseline population.

Conclusions. DI appears to be a key mechanism in the formation of distant companions with masses ranging from low-mass stars down to the planetary regime, contributing, however, only marginally to planetary mass objects inside of 100 AU. Our results are sensitive to a number of physical processes, which are not completely understood. Two of them, gas accretion and clump-clump collisions, are particularly important and need to be investigated further. Magnetic fields and heavy-element accretion have not been considered in our study, although they are also expected to affect the inferred population. We suggest acknowledging the importance of the gravito-turbulent phase, which most protoplanetary discs experience. Exploring hybrid DI – core accretion scenarios, and quantitative comparisons of theory and observations will improve our understanding of star and planet formation.