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In young star clusters, scientists often observe so-called wandering planetary mass objects - celestial bodies that are heavier than rocky planets but lighter than stars. They are not gravitationally bound to any star and drift freely in space. For a long time, scientists have argued about their origin. Finally, researchers are getting closer to the answer. Planetary mass objects (PMOs) or planemes - are neither stars nor planets. They are often called cosmic "nomads" because many of them are "loners", not gravitationally bound to any star, and therefore drift freely in space. The mass of such objects usually does not exceed 13 Jupiter masses. These bodies are mainly found in "stellar nurseries" - young clusters like
Orion's Trapezium.
Although planetary-mass objects are not uncommon in space, their origin has puzzled scientists for decades. According to one version, PMOs may be “failed stars” that lacked the mass to initiate fusion reactions in the manner of brown dwarfs or stars. According to another, PMOs are orphan planets ejected from their host systems in “gravitational fights.” However, none of these hypotheses can explain, for example, 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 with them.
An international team of astronomers led by Deng Hongping (Deng Hongping) from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences. Using computer programs, the scientists recreated conditions in star clusters and simulated collisions between two circumstellar disks—rings of rotating gas and dust that surround young stars.
When the two disks approach each other at a distance of 300–400 astronomical units (an astronomical unit is the distance from the Earth to the Sun) and at a speed of 2–3 kilometers per second, their gravity stretches and compresses the gas, creating “tidal bridges”—elongated structures of matter. Over time, these “bridges” collapse into dense gas “threads,” which then break up into clumps. When the mass of the clumps reaches a critical value, they form PMOs with a mass about 10 times that of Jupiter.
The simulations showed that about 14 percent of PMOs are formed in pairs or triplets, and the distance between these bodies is 7–15 astronomical units. This allows us to understand why in some star clusters scientists observe binary systems of planetary objects. In dense star clusters, for example, in the same Trapezium of Orion, such collisions of disks 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 regular planets. They form from material in the outer regions of circumstellar disks, where there are fewer heavy elements, which makes their chemical composition unique. Unlike orphan planets ejected from their host systems, planetary-mass objects are born with stars and can sometimes move in synchrony with them. In addition, PMOs are left behind as gaseous disks that can reach 200 astronomical units in diameter. Theoretically, planets and moons could form in such disks as mini-versions of the solar system.
The discovery by Hongping's team showed that, with a high probability, PMOs are not a by-product of the evolution of stars or planets, but the result of gravitational "battles" of circumstellar disks. To verify the correctness of their hypothesis, scientists plan to study the chemical composition of PMOs and surrounding disks using the James Webb Space Telescope. Hongping also intends to test his hypothesis on data from other young clusters. If the scientists' conclusions are confirmed, PMOs could become a new class of astronomical objects. The scientific work is published in the journal Science Advances.
