How did the wandering planets form and what is their origin? Has one of these wandering planets ever hit Earth in the past 100 million years?

The origin of planetary mass objects (PMOs) wandering in young star clusters remains enigmatic, especially when they come in pairs. They could represent the lowest-mass object formed via molecular cloud collapse or high-mass planets ejected from their host stars. However, neither theory fully accounts for their abundance and multiplicity. Here, we show via hydrodynamic simulations that free-floating PMOs have a unique formation channel via the fragmentation of tidal bridge between encountering circumstellar disks. This process can be highly productive in density clusters like Trapezium forming metal-poor PMOs with disks. Free-floating multiple PMOs also naturally emerge when neighboring PMOs are caught by mutual gravity. PMOs may thus form a distinct population different from stars and planets.Free-floating planetary-mass objects (PMOs) with masses below the deuterium burning limit (13 Jupiter Mass, !!) are discovered via both direct imaging (1,2) and microlensing (3,4). Isolated PMOs that are more massive than Jupiter, lying at the border between stars and planets, are particularly interesting. Such PMOs glowing in infrared were frequently observed in nearby young star clusters by the James Webb Space Telescope (JWST) (5-8). Are they representatives of the low-mass end members of the star formation process or unlucky giant planets exiled by their parent stars? Neither star-like nor planet-like Stars formed by molecular cloud collapse follow characteristic mass functions. However, in the 10-20 !! mass range, isolated objects can be excessively abundant, as observed in the Trapezium cluster (9), and free-floating PMOs in the Upper Scorpius stellar association are up to 7 times more abundant than the mass functions’ prediction (2). As a result, an extra formation channel is required to explain the rich PMO population. In addition, free-floating multiple PMOs and candidates were reported with projected separations ranging from several to hundreds of astronomical units (au) (5,7,10-12). The high multiplicity (~ 9%) of PMOs in the Trapezium cluster, though needs further confirmation, defies a star-like origin since stellar multiplicity decays with stellar mass, and the extrapolated wide binary fraction for PMOs would be close to zero (13).Nor are they mature planets ejected after dynamic interactions. PMOs formed as ejected planets would have distinctive spatial and kinematic distributions to that of stars, contrasting with the PMOs’ distribution in NGC 1333 (14). Dynamic simulations of the Trapezium cluster also suggest the concurrent formation of PMOs with stars (15), further supported by the prevalence of gaseous disks around PMOs (16-18). To explain free-floating PMOs via planet ejection begs knowledge of the population of wide-orbit giant planets in the first place, which is poorly constrained. Nevertheless, to explain the PMO population in the Trapezium cluster, an unrealistically high occurrence rate of wide-orbit (>100 au) giant planets is required (19,20) in contrast with observations (4,21). Here, we propose a scale-down version of filament fragmentation, the modern theory of star formation (22), as a possible means to form free-floating PMOs, including multiples. When the mass per unit length (line mass) of filamentary molecular clouds is beyond a critical value, ( (here $) is the sound speed and % is the gravitational constant), they fragment on a scale roughly four times the filaments’ width to form dense cores and eventually stars (23). The typical width of such a filament is 0.1 pc with a column density of 0.001 g/cm2 (24). To fragment into PMOs, a 0.001 times thinner (20 au) filament with a column density of around 1000 g/ cm2 is necessary, assuming a comparable temperature. We show with hydrodynamic simulations that such filaments are naturally produced in the tidal bridge connecting two encountering young . circumstellar disks. The filament further fragments into PMOs, sometimes forming binary and even triple PMOs (Figure 1). From disk encounters to dense filaments and cores Many young circumstellar disks are prone to instabilities due to the self-gravity of disk gas (25,26), potentially leading to disk fragmentation and the formation of gaseous planets (27,28). Circumstellar disks appear even more unstable when perturbed by a stellar or circumstellar disk flyby. These flybys can induce the formation of PMOs in disks that are otherwise stable in isolation (29-32). However, brown dwarfs instead of PMOs are often formed in these early studies, and PMOs are only marginally resolved if they are not spurious fragments due to poor resolutions (32-34). We performed a series of hydrodynamics simulations of circumstellar disk encounters with the novel Meshless Finite Mass (MFM) scheme (35) at a mass resolution of 0.0001 Jupiter mass, improved by over an order of magnitude compared to the previous studies. In addition, we focus on the extended tidal bridge, which forms most effectively in near coplanar prograde encounters when the disk’s trunk is in quasi-resonance with the flyby orbit motion (36,37). Informed by observations of the Orion Nebula Cluster (ONC), we construct disk models marginally stable in 100-200 au surrounding low-mass stars of ~0.3 solar mass (38,39). These disks are set onto hyperbolic encounter orbits with periapsis distance &* = 200 − 500 au and velocities at infinity ++ = 1 − 5 km/s (Table S1).