How chaotic planet formation may explain wide-orbit worlds like Planet Nine
by Clarence Oxford
Los Angeles CA (SPX) May 28, 2025
In the remote edges of planetary systems, far beyond the range of traditional planet-forming regions, astronomers have long grappled with the mystery of wide-orbit planets. These gas giants and planetary-mass bodies orbit hundreds to thousands of astronomical units (AU) from their stars, raising fundamental questions about their origins. A new study led by Rice University and the Planetary Science Institute provides compelling answers – and bolsters the case for the elusive Planet Nine.
Using thousands of simulations, the research team demonstrated that wide-orbit planets can naturally emerge during a chaotic early stage of planetary system development. In this phase, stars remain tightly clustered in their birth environments while giant planets within developing systems engage in gravitational skirmishes.
“Essentially, we’re watching pinballs in a cosmic arcade,” said Andre 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.”
The team’s models incorporated various planetary architectures, including systems like our own and exotic configurations with multiple stars. Across scenarios, they consistently observed that internal dynamical instabilities could thrust planets into vast, eccentric orbits, with neighboring stars in the cluster exerting stabilizing influences.
“When these gravitational kicks happen at just the right moment, a planet’s orbit becomes decoupled from the inner planetary system,” said Nathan Kaib, study co-author and senior scientist at the Planetary Science Institute. “This creates a wide-orbit planet – one that’s essentially frozen in place after the cluster disperses.”
Defined as having semimajor axes between 100 and 10,000 AU, wide-orbit planets lie well beyond conventional protoplanetary disk boundaries. These findings suggest that their formation is not anomalous but a frequent result of early system turbulence.
The work offers fresh context for the potential existence of Planet Nine – a hypothetical planet believed to orbit between 250 and 1,000 AU from the Sun. Though unseen, gravitational anomalies in trans-Neptunian objects suggest its influence.
“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.
The study also connects wide-orbit planets to free-floating or rogue planets. These are planetary bodies ejected entirely from their systems, drifting through interstellar space.
“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.”
Central to the study is the concept of “trapping efficiency” – the probability that a planet flung outward remains gravitationally bound. Solar system-like configurations had the highest efficiency rates, up to 10%, while other setups were far less effective.
“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.”
The study points to new observational frontiers. It identifies high-metallicity stars already known to host gas giants as promising candidates for direct imaging of wide-orbit planets. The upcoming Vera C. Rubin Observatory, with its deep-sky capabilities, may play a pivotal role in this search – potentially detecting Planet Nine or closing the case on 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.
Research Report:Very-wide-orbit planets from dynamical instabilities during the stellar birth cluster phase
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