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. 2025 Feb 28;11(9):eadu6058.
doi: 10.1126/sciadv.adu6058. Epub 2025 Feb 26.

Formation of free-floating planetary mass objects via circumstellar disk encounters

Zhihao Fu  1   2 , Hongping Deng  2 , Douglas N C Lin  3   4 , Lucio Mayer  5
Affiliations

Formation of free-floating planetary mass objects via circumstellar disk encounters

Zhihao Fu et al. Sci Adv. .

Abstract

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 bridges between encountering circumstellar disks. This process can be highly productive in dense clusters like Trapezium forming metal-poor PMOs with disks. Free-floating multiple PMOs also naturally emerge when neighboring PMOs are caught by their mutual gravity. PMOs may thus form a distinct population that is fundamentally different from stars and planets.

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Figures

Fig. 1.
Fig. 1.. The formation of binary PMOs via circumstellar disk encounters.
The upper panel shows the density map (2000 au × 1120 au) in a logarithmic scale for an example of a simulation (model rp400v2.65 of table S1). The lower panels enlarge a region of 200 au × 112 au to show the evolution of the binary PMOs, sink particles in white with exaggerated radii, and how they emerge within the dense filament created by the encounter (see movie S1).
Fig. 2.
Fig. 2.. The formation of compact cores within dense filaments under different encounter conditions.
(A to D) show the central region gas column density in g/cm2 in a logarithmic scale for representative simulations in table S1. They have a series of dense cores comparable to the Fig. 1 insert panel at 4392 years. (E) The encounter parameter space that forms FFOs via filament collapse within red curves (table S1); the background contours show the peak angular speed of different flybys divided by the Keplerian frequency at 100 au. (F) Line mass of precursors of the (A) to (D) filaments (~400 years earlier; see also Fig. 1) normalized to the critical line mass for stability (see main text).
Fig. 3.
Fig. 3.. Disks around free-floating PMOs.
(A and B) Similar column density maps like Fig. 1, centered on free-floating PMOs at the end of two simulations (table S1). The PMOs can have extended disks, and the PMO disk in (B) is lopsided because of perturbations from the other PMO (to the upper right, not shown here) in this binary system (see another binary in Fig. 1).
See this image and copyright information in PMC

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