There could be a ninth planet in our Solar System after all — and no, it’s not Pluto. Instead, it’s the theoretical Planet X, also called Planet Nine, a wide-orbit planet that would circle the Sun from far beyond Neptune, and far beyond Pluto, too.
Planet X has been hypothesized for years, but how such a planet could have appeared in the outer Solar System has long stumped scientists. A new study in Nature Astronomy reveals a novel theory, however. The research suggests that wide-orbit planets (such as Planet X) arise in the early evolution of their planetary systems, when these systems are still inside their stars’ crowded birth clusters. In these tight, chaotic settings, stars and their planets are fighting for space, and sometimes planets are sent flying.
“Essentially, we’re watching pinballs in a cosmic arcade,” said André Izidoro, a study author and an assistant professor at Rice University, according to a press release.
Read More: Our Solar System May Have a New Dwarf Planet Orbiting Even Farther Than Pluto
Crowded Clusters and Far-Flung Planets
Though some planets orbit their stars closely, there are some planets that circle their stars from afar, from hundreds or thousands of astronomical units (AU) away.
To figure out how these wide-orbit planets find themselves stuck so far away, researchers from Rice University and the Planetary Science Institute simulated the early evolution of thousands of planetary systems, including those that surround one star and two stars, as well as those with a combination of gas and ice giants.
They started their simulations when these systems were crammed inside their stars’ birth clusters, before their stars spread to more spacious corners of the universe. In these clusters, some planets were “scattered” away from their stars by the gravitational influence of the other planets in their planetary systems, only to be stabilized by the gravitational influence of the other stars in the cluster.
This removed them from the inner portion of their own planetary systems, and “trapped” them in the outer portion, where they remained in wide orbits after the cluster split apart.
“When these gravitational kicks happen at just the right moment, a planet’s orbit becomes decoupled from the inner planetary system,” said Nathan Kaib, another study author and a senior scientist at the Planetary Science Institute, according to the release. “This creates a wide-orbit planet — one that’s essentially frozen in place after the cluster disperses.”
Read More: A Map to Planet Nine: Hunting Our Solar System’s Most Distant Worlds
A Theory For Theoretical Planet X
The researchers say that their simulations suggest how Planet X could have been scattered and trapped in our own Solar System, resulting in a wide orbit. Indeed, the simulations found that the growth and gravitational influences of Uranus and Neptune increased the chances that a planet could have been sent to the outer Solar System in its early evolution.
“There is up to a [40 percent] chance that a Planet Nine-like object could have been trapped during that time,” Izidoro said in the release.
The simulations also showed that some scattered planets — called free-floating planets — are not trapped and are instead sent out of their planetary systems.
Though the majority of planetary systems scatter their planets without trapping them, the portion of systems with wide-orbit planets isn’t insignificant.
“We expect roughly one wide-orbit planet for every thousand stars,” Izidoro said in the release. “That may seem small, but across billions of stars in the galaxy, it adds up.”
According to the researchers, the results explain how Planet X could have acquired its wide orbit, even though its existence still awaits confirmation. Such a confirmation could come from the Vera C. Rubin Observatory, which is expected to illuminate some of the farthest-flung objects in our Solar System as soon as July 2025, when it is currently scheduled to start its operations.
“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 in the release.
Read More: New Telescope Could Potentially Identify Planet X
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Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
