Astronomers have long been puzzled by mysterious, planet-sized objects drifting freely through space, unattached to any star.
These cosmic wanderers, called planetary-mass objects (PMOs), weigh less than 13 times the mass of Jupiter and have been spotted in young star clusters, such as the Trapezium Cluster in Orion.
Scientists once believed they were either failed stars or planets that had been ejected from their solar systems.
However, new research suggests that PMOs form in a completely different way—through the chaotic interactions of young stars and their surrounding gas disks.
An international team of astronomers, including researchers from the University of Zurich (UZH), has used advanced computer simulations to understand how PMOs come to exist.
The study shows that these objects are not ejected planets but instead form naturally during the early stages of star cluster evolution.
Dr. Lucio Meyer from UZH, the lead researcher, explained, “PMOs don’t fit into the usual categories of stars or planets. Our simulations show that they emerge through a completely different process.”
Young stars are often surrounded by rotating disks of gas and dust, known as circumstellar disks. The new research shows that when two of these disks pass close to each other, their powerful gravitational forces pull and stretch the gas into long, thin structures called “tidal bridges.”
These bridges eventually collapse into dense filaments, which then break apart into smaller, compact gas clouds.
If these clouds reach a critical mass—about 10 times the mass of Jupiter—they become planetary-mass objects. Some of these PMOs even form in pairs or groups, which explains why astronomers often find PMO binaries in star clusters.
In dense regions like the Trapezium Cluster, hundreds of PMOs could be created through these interactions.
Unlike planets that orbit stars, PMOs form independently but in the same environment as young stars. They inherit their material from the outer parts of circumstellar disks, making them different from both traditional planets and stars.
Interestingly, PMOs tend to move in sync with the stars in their birth cluster, unlike planets that are violently ejected from solar systems.
Many PMOs also retain a surrounding gas disk, meaning they could potentially form their own moons or even smaller planets. This raises fascinating possibilities about the diversity of planetary systems in the universe.
“This discovery changes how we think about the formation of objects in space,” said Dr. Meyer. “PMOs might represent a third category of celestial bodies, forming not from the raw material of star-forming clouds or through normal planet-building processes, but through the gravitational chaos of disk collisions.”
This new understanding of PMOs sheds light on the complex and dynamic environments of young star clusters, showing that even chaos can create new worlds.
