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.
For the study, the team ran thousands of simulations involving different planetary systems embedded in realistic star cluster environments.
“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.
Interestingly, the study also ties wide-orbit planets to the growing population of free-floating, or “rogue,” planets—worlds ejected from their systems entirely.
Mysterious gas giants and planetary masses orbit their stars in silence, sometimes thousands of astronomical units (AU) away, in the cold, dark outskirts of planetary systems far beyond the reach of the known planets. How these “wide-orbit” planets, such as the elusive Planet Nine that has been hypothesized in our own solar system, could have formed has long baffled scientists. It looks like an astronomy team has finally figured out the solution.
Using intricate simulations, scientists from the Planetary Science Institute and Rice University demonstrated in a recent study published in Nature Astronomy that wide-orbit planets are not anomalies but rather the inevitable result of a chaotic early stage in the formation of planetary systems. It happens when planets are fighting for space in crowded, turbulent systems and stars are still densely packed in their birth clusters.
André Izidoro, the study’s lead author and an assistant professor of Earth, environmental, and planetary sciences at Rice, described the experience as “basically, we’re watching pinballs in a cosmic arcade.”. Some are hurled far from their star when giant planets scatter one another due to gravitational interactions. Those planets are trapped in incredibly wide orbits rather than being ejected if the timing and environmental conditions are ideal. “..”.
The team used thousands of simulations with various planetary systems embedded in realistic star cluster environments to conduct the study. They simulated a wide range of scenarios, from more unusual ones like those with two suns to more exotic ones like our solar system, which is made up of a mixture of gas and ice giants. They identified a recurrent pattern: Internal instabilities often forced planets into broad, eccentric orbits, which were subsequently stabilized by the gravitational pull of neighboring stars in the cluster.
According to Nathan Kaib, a senior scientist and senior education and communication specialist at the Planetary Science Institute, “a planet’s orbit becomes decoupled from the inner planetary system when these gravitational kicks happen at just the right moment.”. When the cluster disperses, this produces a planet in wide orbit that is effectively frozen in place. “.
Wide-orbit planets, according to the researchers, are planets with semimajor axes between 100 and 10,000 AU, which is a far cry from the range of most conventional planet-forming disks.
The results may contribute to the resolution of the long-standing mystery surrounding Planet Nine, a fictitious planet thought to orbit our sun between 250 and 1,000 astronomical units. The strange orbits of a number of trans-Neptunian objects suggest its existence, despite the fact that it has never been directly observed.
Izidoro stated, “Our simulations show that there is up to a 40 percent chance that a Planet Nine-like object could have been trapped during the two specific instability phases that the early solar system underwent—the growth of Uranus and Neptune and the later scattering among gas giants.”.
Curiously, the study also links the increasing number of free-floating, or “rogue,” planets—worlds that have been completely ejected from their systems—to wide-orbit planets.
“Not all scattered planets are fortunate enough to become trapped,” Kaib remarked. Ultimately, the majority are launched into interstellar space. We can relate the planets we see on broad orbits to those we find roving alone in the galaxy, however, because of the speed at which they become trapped. “..”.
At the heart of the research is the idea of “trapping efficiency”—the probability that a dispersed planet will stay connected to its star. Systems that resemble the solar system are especially effective, according to the researchers, with trapping probabilities ranging from 5% to 10%. Other systems had significantly lower efficiencies, such as those made up solely of circumbinary planets or ice giants.
Izidoro stated, “We expect roughly one wide-orbit planet for every thousand stars,”. Even though it might not seem like much, it adds up over the galaxy’s billions of stars. “.”.
The study also finds new and promising targets for exoplanet hunters. It implies that high-metallicity stars that are already home to gas giants are the most likely to have wide-orbit planets, which makes them excellent candidates for deep imaging campaigns.
Additionally, the researchers pointed out that if Planet Nine is real, it might be found shortly after the Vera C. The Rubin Observatory starts up and runs smoothly. The observatory is anticipated to make a substantial contribution to the hunt for far-off solar system objects with its unmatched capacity to conduct in-depth and detailed sky surveys, raising the possibility of either finding Planet Nine or offering the proof required to rule it out.
In addition to improving the chances of discovering Planet Nine, Izidoro stated, “As we improve our knowledge of where to search and what to look for, we’re opening a new window into the architecture and evolution of planetary systems throughout the galaxy.”.
