Disc fragmentation – II. Ejection of low-mass free-floating planets from growing binary systems

ABSTRACT Observations indicate that disc fragmentation due to gravitational instability (GI) is the likely origin of massive companions to stars, such as giant planets orbiting M-dwarf stars, brown dwarf companions to FGK stars, and binary stars with separations smaller than $\sim 100$ au. Additionally, we have recently showed that disc fragmentation in young rapidly evolving binary systems ejects an abundant population of massive Jupiter-mass free-floating planets (FFPs). In this model, a massive disc around an initially single protostar undergoes GI and hatches a number of fragments; the most massive oligarch grows by runaway accretion into the secondary star. As the system rearranges itself from a single to a binary star configuration, a dramatic ‘pincer movement’ by the binary ejects planets through dynamical interactions with the stars. Here we propose that the same scenario applies to an even more abundant population of smaller FFPs discovered by the microlensing surveys. Although disc fragmentation is usually believed to form only massive objects, several pathways for forming small core-dominated planets at separations of tens of au exist. We present results from three complementary simulation approaches, all of which confirm planet ejection efficiency as high as $\sim 50$ per cent for secondaries more massive than $\sim 10$ per cent of the primary star mass. On the other hand, Jovian mass planets migrate through the region of tens of au too rapidly to eject planets from that region. We discuss implications of this scenario, concluding that microlensing FFPs may be the most convincing evidence yet that disc fragmentation forms low-mass ($M_{\rm p}\ll 1{\, {\rm M}_{\rm J}}$) planets.