A new study from astronomers at Lund University in Sweden suggests that Planet 9—a theoretical planet orbiting in the outer reaches of our solar system—may not have formed around our sun. Instead, their model suggests that the planet, if it exists, was probably captured from another star when the sun had just begun its life 4.5 billion years ago.
There is strong evidence to suggest that something about 10 times the mass of Earth is orbiting the sun at an incredible distance. Astronomers can measure this object's gravitational influence on the orbit of a group of trans-Neptunian objects, though we have not actually seen Planet 9.
If Planet 9 exists, it makes sense that it could have formed around another star and then been captured by the gravitational pull of our sun. For one thing, the eight confirmed planets and dozens of dwarf planets, asteroids, and other objects orbiting the sun all formed in relatively close orbits, whereas planet 9 is suspected to orbit the sun at a distance 10 times greater than Neptune's orbit. It seems strange that there would be enough gas and debris out there to accrete into a planet.
If the sun stole Planet 9 from another star system, it would have occurred at a time when the sun had just formed in a stellar cluster of dust and gas, near other newborn stars. Because the sun was closer to other stars 4.5 billion years ago, it theoretically could have ripped a planet away from its original orbit.
"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," said Alexander Mustill, an astronomer at Lund University, in a press release. "When the sun later departed from the stellar cluster in which it was born, Planet 9 was stuck in an orbit around the sun."
It will take much more research to confirm Mustill's theory, especially considering that we have yet to lay eyes on Planet 9. But if he is correct, the findings could help astronomers develop new understandings of how planets form and interact with their young host stars.
Source: Lund University