All planets orbit a star; in the case of our own system, it’s the Sun. This was the long-held view of the orderly conditions in the universe. However, in reality, the circumstances of planets are far more chaotic. This became evident when the first loners were caught on their starless journeys. It quickly became clear that there must be at least some planets not bound to any star, wandering alone through the dark cosmos.
Recently, the situation has dramatically reversed: a series of observations and studies have concluded that conventional planetary systems might actually be the exception. “Rogue Planets”—the English term for these ghost planets—may even be the norm.
Mystery of Origin
Two recent studies by researchers from NASA and Osaka University have concluded that our Milky Way is populated by trillions of planetary vagabonds. Some experts even consider it plausible that these planets could harbor life.
Where these loners come from remains unclear. Two theories are being considered, and both are likely true, though opinions differ on their relative importance. One hypothesis suggests that such worlds, like all other planets, were born in the protoplanetary disk around a new star and were then flung into solitude through gravitational interactions.
According to the second theory, these ghost planets formed in interstellar space from local clumps of gas and dust. They formed like stars but never gained enough mass for the fusion reaction that would make them shine. These would be large planets that never had a parent star.
Six Free Spirits
However, all of this is still speculation, as there are only a few concrete observations—understandably so, since a ghost planet without a parent star and sibling planets to interact with gravitationally is difficult to spot in the darkness of space. Better data than what ground-based telescopes can currently provide is needed.
The James Webb Space Telescope can provide this much-needed data, and indeed, it recently succeeded in observing six free-floating exoplanets. These newly discovered ghost worlds also provide the first strong evidence of the processes that led to their existence as loners: most of the Milky Way’s vagabonds likely emerged alone, like stars, from a cloud of gas and dust.
Young Nebula in Focus
The researchers are excited about these findings, particularly because they could answer some open questions about the formation of stars and planets. “We are exploring the limits of the star formation process,” explained Adam Langeveld, an astrophysicist at Johns Hopkins University and lead author of the study, which will appear in the Astrophysical Journal and has been pre-published on the arXiv.org preprint server.
The Webb findings come from a survey of the young nebula NGC1333, a star-forming region made of interstellar gas and dust about a thousand light-years away in the Perseus constellation. The newly discovered worlds are gas giants, five to ten times more massive than Jupiter. While this sounds enormous, they are among the least massive free-floating planetary objects ever observed. They are not classified as brown dwarfs, which are significantly more massive.
Like Large Conventional Planets
“We used Webb’s unprecedented sensitivity at infrared wavelengths to search for the faintest members of a young star cluster, seeking to address a fundamental question in astronomy: How light an object can form like a star?” said Ray Jayawardhana, also a lead author of the study. “It turns out the smallest free-floating objects that form like stars overlap in mass with giant exoplanets circling nearby stars.”
No objects with a mass of less than five Jupiters were detected, even though the telescope is sensitive enough for such detections. The most fascinating of these starless objects is also the lightest, with an estimated mass of five Jupiters (about 1,600 Earth masses). This world is surrounded by a dust-rich disk, confirming the suspicion that it formed like a star.
Tiny Planetary Systems
Such disks are also the “birthplace” of planets. The observation suggests that these ghost worlds could be surrounded by smaller planets or moons, so to speak. “This might be a nursery of a miniature planetary system, on a scale much smaller than our solar system,” said co-author Aleks Scholz of the University of St Andrews.
The researchers also observed something else fascinating: they discovered a brown dwarf with a planetary-mass companion—a rare find that challenges theories about the formation of binary star systems. “It’s likely that such a pair formed the way binary star systems do, from a cloud fragmenting as it contracted,” Jayawardhana said. “The diversity of systems that nature has produced is remarkable and pushes us to refine our models of star and planet formation.”
Outdated Categories
Such findings force the scientific community to reconsider its cherished classification system. According to the research team, these free-roaming worlds blur the categories of celestial bodies, as their masses overlap with those of star-bound gas giants and brown dwarfs. Furthermore, the observations show that these ghost planets are quite common: according to Webb’s data, they make up about ten percent of the celestial bodies in the observed star cluster.