In the cold, dark outskirts of planetary systems far beyond the reach of the known planets, mysterious gas giants and planetary masses silently orbit their stars — sometimes thousands of astronomical units (AU) away. For years, scientists have puzzled over how these “wide-orbit” planets, including the elusive Planet Nine theorized in our own solar system, could have formed. Now, a team of astronomers may have finally found the answer.
In a new study published in Nature Astronomy, researchers from Rice University and the Planetary Science Institute used complex simulations to show that wide-orbit planets are not anomalies but rather natural by-products of a chaotic early phase in planetary system development. This phase occurs while stars are still packed tightly in their birth clusters and planets are jostling for space in turbulent, crowded systems.
“Essentially, we’re watching pinballs in a cosmic arcade,” said André Izidoro, assistant professor of Earth, environmental and planetary sciences at Rice and the study’s lead author. “When giant planets scatter each other through gravitational interactions, some are flung far away from their star. If the timing and surrounding environment are just right, those planets don’t get ejected, but rather they get trapped in extremely wide orbits.”
For the study, the team ran thousands of simulations involving different planetary systems embedded in realistic star cluster environments. They modeled a variety of conditions, from systems like our solar system with a mix of gas and ice giants to more exotic systems including those with two suns. What they discovered was a recurring pattern: Planets were frequently pushed into wide, eccentric orbits by internal instabilities, then stabilized by the gravitational influence of nearby stars in the cluster.
“When these gravitational kicks happen at just the right moment, a planet’s orbit becomes decoupled from the inner planetary system,” said study co-author Nathan Kaib, senior scientist and senior education and communication specialist at the Planetary Science Institute. “This creates a wide-orbit planet — one that’s essentially frozen in place after the cluster disperses.”
The researchers define wide-orbit planets as having semimajor axes between 100 and 10,000 AU — distances that place them far beyond the reach of most traditional planet-forming disks.
The findings could help explain the long-standing mystery of Planet Nine, a hypothetical planet believed to orbit our sun at a distance of 250 to 1,000 AU. Though it has never been directly observed, the odd orbits of several trans-Neptunian objects hint at its presence.
“Our simulations show that if the early solar system underwent two specific instability phases — the growth of Uranus and Neptune and the later scattering among gas giants — there is up to a 40% chance that a Planet Nine-like object could have been trapped during that time,” Izidoro said.
Interestingly, the study also ties wide-orbit planets to the growing population of free-floating, or “rogue,” planets — worlds ejected from their systems entirely.
“Not every scattered planet is lucky enough to get trapped,” Kaib said. “Most end up being flung into interstellar space. But the rate at which they get trapped gives us a connection between the planets we see on wide orbits and those we find wandering alone in the galaxy.”
This concept of “trapping efficiency” — the likelihood that a scattered planet remains bound to its star — is central to the study. The researchers found that solar system-like systems are particularly efficient with trapping probabilities of 5-10%. Other systems, like those composed only of ice giants or circumbinary planets, had much lower efficiencies.
“We expect roughly one wide-orbit planet for every thousand stars,” Izidoro said. “That may seem small, but across billions of stars in the galaxy, it adds up.”
Moreover, the study identifies promising new targets for exoplanet hunters. It suggests that wide-orbit planets are most likely to be found around high-metallicity stars that already host gas giants, making these systems prime candidates for deep imaging campaigns. The researchers also noted that if Planet Nine exists, it could be discovered soon after the Vera C. Rubin Observatory becomes fully operational. With its unparalleled ability to survey the sky in depth and detail, the observatory is expected to significantly advance the search for distant solar system objects, increasing the likelihood of either detecting Planet Nine or providing the evidence needed to rule out its existence.
“As we refine our understanding of where to look and what to look for, we’re not just increasing the odds of finding Planet Nine — we’re opening a new window into the architecture and evolution of planetary systems throughout the galaxy,” Izidoro said.