How do rogue planetary-mass objects, celestial bodies that fall between planets and stars in size, come into existence?
An international team of astronomers, including researchers from the University of Zurich, has tackled this mystery using advanced simulations. Their findings reveal that these elusive objects are closely tied to the turbulent interactions within young star clusters, where chaotic gravitational forces and disk collisions play a key role in their formation.
Cosmic Nomads: The Mystery of Planetary-Mass Objects
Planetary-mass objects (PMOs) are cosmic nomads, drifting freely through space without being tied to a star. These objects are smaller than 13 times the mass of Jupiter and have been found in large numbers within young star clusters like the Trapezium Cluster in Orion (see image above). Their origins, however, have long puzzled scientists. Until now, many theories suggested that PMOs were either failed stars or planets that had been ejected from their original solar systems.
Now, an international team of astronomers, in collaboration with the University of Zurich (UZH), has used advanced simulations to offer a new explanation. Their findings suggest that PMOs can form naturally through the violent interactions between disks of gas and dust surrounding young stars. “PMOs don’t fit neatly into existing categories of stars or planets,” said Lucio Meyer from UZH, corresponding author of the study. “Our simulations show they are probably formed by a completely different process.”
How Disks Collide to Create PMOs
To test this idea, researchers from the University of Zurich, the University of Hong Kong, the Shanghai Astronomical Observatory, and the University of California Santa Cruz ran high-resolution hydrodynamic simulations. They modeled close encounters between two circumstellar disks – rotating rings of gas and dust that form around young stars. When these disks pass near each other, their gravitational pull distorts the gas, creating long, stretched-out structures known as “tidal bridges.”
The Birth of Free-Floating Worlds
The simulations showed that these bridges collapse into dense filaments, which then break apart into compact cores. When the mass of these filaments reaches a critical threshold, they form PMOs, typically with masses around 10 times that of Jupiter. The researchers also found that up to 14% of PMOs form in pairs or small groups, which may explain why some clusters contain a high number of PMO binaries. In star-forming regions like the Trapezium Cluster, where disk encounters are frequent, this process could generate hundreds of PMOs.
Why PMOs Are Unique
PMOs form along stars, inheriting material from the outer edges of circumstellar disks. PMOs move in synchrony with the stars in their host cluster, unlike ejected planets. Many PMOs retain gas disks, suggesting the potential for moon or even planet formation around these nomads.
“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 the raw material of star-forming clouds or via planet-building processes, but rather from the gravitational chaos of disk collisions.”
Explore Further: Scientists Just Discovered a Violent New Way Rogue Planets Are Made
Reference: “Formation of free-floating planetary mass objects via circumstellar disk encounters” by Zhihao Fu, Hongping Deng, Douglas N. C. Lin and Lucio Mayer, 26 February 2025, Science Advances.
DOI: 10.1126/sciadv.adu6058
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