Disc fragmentation rarely forms planetary-mass objects

TL;DR

This study investigates the likelihood of planetary-mass objects forming through disc fragmentation at large radii and finds such objects are rare, with observed properties aligning more with other formation mechanisms.

Contribution

The paper provides new constraints on the frequency of planetary-mass objects formed via disc fragmentation by analyzing their dynamical evolution under stellar perturbations.

Findings

Disc fragmentation at radii >50 au rarely produces planetary-mass objects.

Objects formed via disc fragmentation are likely scattered and not observed as stable planets.

Observed exoplanets in relevant regions are more consistent with other formation processes.

Abstract

It is now reasonably clear that disc fragmentation can only operate in the outer parts of protostellar discs ($r > 50$ au). It is also expected that any object that forms via disc fragmentation will have an initial mass greater than that of Jupiter. However, whether or not such a process actually operates, or can play a significant role in the formation of planetary-mass objects, is still unclear. We do have a few examples of directly imaged objects that may have formed in this way, but we have yet to constrain how often disc fragmentation may actually form such objects. What we want to consider here is whether or not we can constrain the likely population of planetary-mass objects formed via disc fragmentation by considering how a population of objects at large radii ($a > 50$) au - if they do exist - would evolve under perturbations from more distant stellar companions. We find that there is a specific region of parameter space to which such objects would be scattered and show that the known exoplanets in that region have properties more consistent with that of the bulk exoplanet population, than with having been formed via disc fragmentation at large radii. Along with the scarcity of directly-imaged objects at large radii, our results provide a similar, but independent, constraint on the frequency of objects formed via disc fragmentation.