Astronomers have revealed how cosmic "wanderers" are born.

Planetary mass objects (PMOs) or planemos are neither stars nor planets. They are often referred to as cosmic "wanderers" because many of them are "loners," gravitationally unbound to any celestial body, thus freely drifting through space. The mass of such objects typically does not exceed 13 times that of Jupiter. These bodies are predominantly found in "stellar nurseries" — young clusters like the Orion Nebula.

Although planetary mass objects are not rare in the cosmos, their origin has puzzled scientists for decades. According to one theory, PMOs could be "failed stars" that lacked sufficient mass to initiate nuclear fusion, similar to brown dwarfs or stars. Another theory suggests that PMOs are orphan planets ejected from their home systems due to "gravitational scuffles."

However, none of these hypotheses can explain, for instance, why there are so many PMOs in space, why they sometimes form pairs, and move synchronously with stars in clusters — as if they were born together.

To address these questions, an international team of astronomers led by Deng Hongping from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences attempted to investigate. Using computer programs, the scientists recreated conditions in star clusters and simulated collisions between two circumstellar disks — rotating rings of gas and dust surrounding young stars.

When two disks come close together at a distance of 300–400 astronomical units (an astronomical unit is the distance from the Earth to the Sun) and at speeds of 2–3 kilometers per second, their gravity stretches and compresses the gas, creating "tidal bridges" — elongated structures of material. Over time, these "bridges" collapse into dense gas "threads," which then break apart into clumps. When the mass of these clumps reaches critical values, PMOs form with masses approximately ten times that of Jupiter.

Modeling indicated that about 14 percent of PMOs are formed in pairs or triplets, with distances between these bodies ranging from 7 to 15 astronomical units. This helps to understand why scientists observe double systems of planetary mass objects in some star clusters.

In dense star clusters, such as the Orion Nebula, such disk collisions occur frequently. Therefore, hundreds of PMOs can be born there, which explains why astronomers find so many of these bodies in space.

According to the analysis conducted by Hongping's team, PMOs do not form like ordinary planets. They arise from material in the outer regions of circumstellar disks, where there are fewer heavy elements, making their chemical composition unique. Unlike orphan planets that have been ejected from their home systems, planetary mass objects are born together with stars and can sometimes move synchronously with them.

Furthermore, PMOs retain gas disks that can reach up to 200 astronomical units in diameter. Theoretically, planets and satellites could form in such disks — akin to mini-versions of the Solar System.

The findings of Hongping's team indicate that PMOs are likely not a byproduct of stellar or planetary evolution, but rather the result of gravitational "battles" within circumstellar disks. To validate their hypothesis, the scientists plan to study the chemical composition of PMOs and their surrounding disks using the James Webb Space Telescope. Hongping also intends to test his hypothesis using data from other young clusters. If the scientists' conclusions are confirmed, PMOs could represent a new class of astronomical objects.

The scientific work has been published in the journal Science Advances.