We live in an age of exoplanet discovery, and have discovered several thousand planets orbiting distant stars. These discoveries hold important lessons about planetary formation and solar system architecture. But we’re also discovering a growing number of rogue planets, also called free-floating planets. These planets aren’t gravitationally bound to any star. What can they teach us?
In the 1990s, scientists realized that gravitational interactions could eject planets from their solar systems, creating rogue planets or free-floating planets. Simulations showed that these ejections could be common, and that multitudes of these planets could be wandering through the galaxy. In 2011, a team of astronomers using the Canada-France-Hawaii Telescope discovered one of these forlorn worlds about 100 light-years away from Earth. Now we know of many more, and some recent estimates say there could be trillions of them in the Milky Way, outnumbering the stars themselves.
In new research, a team of astronomers from Scotland’s St. Andrews University and other institutions examined eight rogue planets with masses between five and ten Jupiter masses. Six of the eight planets are surrounded by dusty disks. This could be a sign that other worlds can form around these drifting planets, compromising a small-scale planetary system.
The research is “Spectroscopy of Free-Floating Planetary-Mass Objects and their disks with JWST,” and is published in The Astronomical Journal. The lead author is Belinda Damian. She is a research fellow in the School of Physics and Astronomy at St. Andrews University in Scotland.
“Free-floating planetary-mass objects (FFPMOs) are known to harbor disks at young ages,” the researchers explain. “Here, we present 1-13 μm spectra for eight young FFPMOs with masses of 5-10 M (at ages of 1-5 Myr), using the NIRSpec and MIRI instruments on the James Webb Space Telescope.” These planets are dim and only emit infrared light, so the JWST is the tool of choice for observing them.
There could be two pathways for planets to become FFPMOs. They could form in the protoplanetary disks around stars and be ejected from their solar systems by gravitational interactions, or they could form from direct collapse of clouds like stars do. In both cases, these objects are not massive enough to undergo hydrogen fusion, the hallmark of a star. Some of the more massive ones could be brown dwarfs, which undergo an initial period of weaker deuterium fusion.
Six of the eight observed planets show signs of warm, dusty disks around them. In young solar systems, this is where planets form. The JWST observations show emissions from silicon grains in the disk, as well as evidence of dust growth and crystallization.
This is a critical aspect of planet formation, as the dust grains bridge the gap between microscopic interstellar dust and large planetesimals that eventually become rocky worlds. Crystalline silicates are particularly important because they’re robust and can better survive the chaotic environment in a disk. Astronomers have found silicate emissions around stars and brown dwarfs before, but this is the first time they’ve been spotted around a rogue planet.
This figure shows some of the spectra from the six objects with silica in their disks. “The variations seen in this figure can be explained by the differences in grain sizes and degree of crystallization,” the authors write. “Amorphous ISM-type silicates cause a strong silicate feature, with a single peak around 9–10 μm, as seen in the three objects at the top of the figure. On the other hand, crystalline-rich silicates typically show two peaks at 9.3 μm (enstatite) and 11.3 μm (forsterite), resulting in a flattened appearance of the feature, as seen in UGC0422+2655 at the bottom of the plot. For the remaining two objects, the 10 μm feature tends to show a transition state between the amorphous submicron-sized silicate grains and the processed crystalline silicates.” Image Credit: Damian et al. 2025. AnJ
Previous research showed that these disks can persist for millions of years, easily long enough for planets to form. That research studied 13 planetary mass objects. “About a third to half of them retain their discs for several million years,” that paper stated. “The long-lived discs identified here may signal that there might be planets around objects, which themselves have masses comparable to giant planets.”
“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,” said principal investigator Dr Aleks Scholz from St. Andrews, also an author of the previous paper. “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 researchers also found significant water absorption features around all of the targets, and CO2 absorption in five of them. There are also hints of differences in atmospheric cloud distribution and overall chemistry. The different shapes and strengths of the silica features among the worlds reflect differences in grain growth and the degree of crystallization. Taken altogether, six of the eight objects seem to be in the very earliest stages of forming smaller, rocky, sibling planets.
In fact, the planet formation process could be hastened around these lower mass objects compared to around brown dwarfs. “We note that processes that dominate the dust growth such as radial drift are expected to be more efficient around very low-mass objects than around brown dwarfs.” the authors write.
Whether or not rogue planets or ejected from solar systems or form from direct collapse, the implication is clear. If they host disks of silica dust that is crystallizing, then there’s a strong reason to believe that planets can form there. There’s no known physical reason that they can’t, but there’s no direct observational evidence that they are. These disks with there crystallizing silica are the best evidence currently.
“These are the lowest-mass isolated objects found so far with silicate and hydrocarbon emission features arising in their disks,” the authors write in their research. “The presence of disks and their characteristics point to the potential for the formation of rocky companions around free-floating planetary-mass objects.”
“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,” said lead author Belinda Damian in a press release. “This means that the formation of planetary systems is not exclusive to stars but might also work around lonely starless worlds.”
Given that there doesn’t seem to be any reason why massive rogue planets can’t form smaller worlds in their disks, the question becomes an observational one. The upcoming Nancy Grace Roman Space Telescope is expected to find around 400 free-floating planetary mass objects, and could revolutionize our understanding of them. If it can provide definitive evidence of free-floating planets forming smaller companions in their disks, then the age of exoplanet discovery will have entered another chapter.