Earlier this year, astrophysicists made waves when they announced a ninth planet ten times bigger than Earth is chilling somewhere in the outer solar system, beyond the Oort cloud. We’ve yet to find this planet, not even a flicker of light, but the evidence gathered so far like the gravitational anomalies exerted on other planets in the solar system is very strong. Even so, some researchers are already debating its origin. One group from Lund University in Sweden says Planet 9 or Planet X, as it’s sometimes called, might actually be an exoplanet, initially formed in another solar system but captured by our sun in an interstellar gravity tug of war.

An artistic rendering of Planet 9, a planet astronomers have not seen but only inferred based on anomalies found in the orbits of distant Kuiper Belt Objects. Photo by Caltech

“It is almost ironic that while astronomers often find exoplanets hundreds of light years away in other solar systems, there’s probably one hiding in our own backyard,” said researcher Alexander Mustill, who was involved in the study.

Mustill investigated the scenario where Planet 9 is in fact a captured planet. Any capture scenario must satisfy three conditions:

• the encounter must be more distant than ∼150 AU to avoid perturbing the Kuiper belt;
• the other star must have a wide-orbit planet (a ≳ 100 au);
• the planet must be captured on to an appropriate orbit to sculpt the orbital distribution of wide-orbit Solar system bodies.

The researchers’ simulation seems to suggest these conditions have been met several times over a 100 million years time frame, assuming there are many Neptune-sized planets in our stellar neighbourhood.

“Planet 9 may very well have been ‘shoved’ by other planets, and when it ended up in an orbit that was too wide around its own star, our sun may have taken the opportunity to steal and capture Planet 9 from its original star,” Mustill explained. “When the sun later departed from the stellar cluster in which it was born, Planet 9 was stuck in an orbit around the sun.”

Credit: Space.com

Planet Nine is thought to have an elongated oval loop orbit around the sun, and it takes anywhere from 10,000 to 20,000 years to complete a full revolution. This means there’s a huge search area, despite astronomers’ best efforts to narrow the search. There’s also the possibility that Planet 9 doesn’t exist, but some really smart people think the odds are in Planet 9’s favor. Someday, we’ll get to the bottom of this. Until then the possibility of a ninth planet stolen from another solar system orbiting inside our own solar system is definitely exciting.

“While the existence of Planet 9 remains unproven, we consider capture from one of the Sun’s young brethren a plausible route to explain such an object’s orbit,” the team wrote in their paper publsihed in the Monthly Notices Letters of the Royal Astronomical Society.