Planets without stars may not be so lonely after all. New research led by astronomers at the University of St Andrews suggests that free-floating giant planets—those adrift in interstellar space—can host their own miniature planetary systems.
Using the unparalleled infrared vision of the James Webb Space Telescope (JWST), the team studied a sample of eight young, isolated planetary-mass objects with masses five to ten times that of Jupiter. Though not bound to any star, these solitary giants appear to possess dusty circumplanetary discs—structures similar to the ones that formed the planets around our Sun.
The findings, published today in The Astronomical Journal mark a significant step in understanding how planetary systems can emerge in the absence of stars.
“These discoveries show that the building blocks for forming planets can be found even around objects that are barely larger than Jupiter and drifting alone in space. This means that the formation of planetary systems is not exclusive to stars but might also work around lonely starless worlds,” says Belinda Damian, the study’s lead author.
Free-floating planets are notoriously difficult to detect, shining faintly in infrared and offering little in the way of visible clues. Yet they hold tantalising hints about the lower limits of star formation and the outer fringes of planetary assembly. Some free floaters are thought to form like stars, from the gravitational collapse of gas clouds. Others may begin life as ordinary planets orbiting stars before being ejected into the cosmic void.
Between August and October 2024, the researchers used JWST’s sensitive infrared instruments to conduct detailed spectroscopic studies of their targets. The data revealed that six of the eight objects were surrounded by warm dust emitting strongly in the infrared—a hallmark of circumstellar discs.
More intriguingly, the team detected emission from silicate grains within those discs, showing signs of grain growth and crystallisation. These processes are believed to be the first stages in the formation of rocky worlds.
“Taken together, these studies show that objects with masses comparable to those of giant planets have the potential to form their own miniature planetary systems,” says Aleks Scholz, the project’s principal investigator. “Those systems could be like the Solar System—just scaled down by a factor of 100 or more in mass and size. Whether or not such systems actually exist remains to be shown.”
The study builds on earlier work by the same group showing that discs around free-floating planetary-mass objects can persist for millions of years—potentially long enough for planets to take shape.
But should we really be calling these hypothetical companions planets, or are they simply moons?
“We don’t really know yet what to call a smaller body orbiting a free-floating object that is similar in mass to a planet,” Scholz told Astronomy Now, “There are people who would rather call them moons, and that’s fine, too. These systems – if they exist – would look more comparable to Jupiter and its moons than to our solar system but a moon is typically ‘something that orbits something that orbits something else’, and that’s not the case here. At some point we need to have a debate what to call them.”
Scholz points out that arguments about nomenclature and definitions usually mean that we have found something entirely new and exciting. Certainly, if confirmed, the existence of scaled-down planetary systems orbiting starless giant planets could open a whole new category in planetary science, challenging assumptions about where—and how—worlds are born.
