Young Star Clusters Spawn Free-Floating Planetary-Mass Objects

Young Star Clusters Spawn Free-Floating Planetary-Mass Objects
by Robert Schreiber
Berlin, Germany (SPX) Feb 28, 2025

Planetary-mass objects (PMOs) are celestial wanderers, freely drifting through space without being bound to any star. Weighing less than 13 times the mass of Jupiter, these objects have been observed in abundance within young star clusters like the Trapezium Cluster in Orion. While their origins have long puzzled scientists, prevailing theories have suggested they might be either failed stars or planets expelled from their solar systems.

Now, an international team of astronomers, in collaboration with the University of Zurich (UZH), has employed advanced simulations to reveal that these enigmatic objects can emerge directly from the interactions of disks surrounding young stars. PMOs don't fit neatly into existing categories of stars or planets, explained Lucio Meyer from UZH, the study's corresponding author. Our simulations show they are probably formed by a completely different process.

How Disk Interactions Generate PMOs

Utilizing high-resolution hydrodynamic simulations, researchers from UZH, the University of Hong Kong, the Shanghai Astronomical Observatory, and the University of California Santa Cruz replicated encounters between circumstellar disks-rotating rings of gas and dust encircling young stars. When these disks pass close to one another, their gravitational forces distort the gas into elongated structures known as tidal bridges.

The simulations demonstrated that these bridges collapse into dense filaments, which then fragment into compact cores. Once these filaments exceed a critical mass threshold, they produce PMOs with sizes reaching approximately 10 times that of Jupiter. The study also found that up to 14% of these objects form in pairs or triples, providing a potential explanation for the high number of PMO binaries in certain clusters. In dense environments like the Trapezium Cluster, frequent disk encounters could lead to the formation of hundreds of PMOs.

The Unique Nature of PMOs

Unlike ejected planets, PMOs form alongside stars, inheriting material from the outer edges of circumstellar disks. They move in sync with stars in their native cluster, rather than being randomly scattered. Many PMOs retain their own gas disks, indicating they could potentially host moons or even planetary bodies.

This discovery partly reshapes how we view cosmic diversity, said co-author Lucio Mayer. PMOs may represent a third class of objects, born not from star-forming clouds or traditional planet-building processes, but rather from the gravitational turmoil of colliding circumstellar disks.

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