Disc fragmentation. I. Ejection of Jupiter-mass Free Floating Planets from growing binary systems

Abstract

Over the past 25 years, observations have uncovered a large population of free-floating planets (FFPs), whose origins remain debated. Massive FFPs (several Jupiter masses or more) may form via gravitational collapse of molecular clouds, similar to stars. Lower-mass FFPs likely originate in planetary systems and are later ejected through dynamical interactions. We show that disc fragmentation in very young stellar binaries may be an abundant source of Jupiter-like FFPs (JFFPs), with masses above 0.3 Jupiter masses. In our model, disc fragmentation at tens to 100 au from the primary star produces gas giants, while fragmentation further out forms a more massive object that will eventually evolve into the secondary star. We present 3D simulations of massive, self-gravitating discs with embedded Jupiter-mass planets and a secondary seed ($5-50 M_J$). Chaotic migration leads to frequent planet-secondary interactions, imparting velocity kicks via gravitational slingshot that usually end in planet ejection. The ejection fraction increases steeply with the secondary-to-primary mass ratio, $q_s$, reaching 0.6 for $q_s > 0.05$. Compared to Core Accretion JFFPs, disc fragmentation JFFPs: (i) form earlier, and may be more abundant in young clusters; (ii) are ejected at much lower velocities; (iii) may retain compact circum-planetary discs. To reproduce the observed abundance of JFFPs, disc fragmentation planets must be in the post-collapse configuration. They must also either form more frequently in binary systems than around single stars, or be frequently disrupted in inner disc regions, as previously suggested in certain versions of the theory.